轉:你從未見過如此詳細的“閉包和作用域鏈”


0.前言

本文適合人群:

  1. 新手小白
  2. 一定JS程度的同學
  3. 閑着過來湊熱鬧的

好吧,最后一個是賣萌,= ̄ω ̄=,廢話不多說了,正文開始。

1. 對象

要理解閉包,首先要先知道閉包的基本原理是什么。

說白了,最常見的閉包就是一個函數嵌套另外一個函數,然后通過突破作用域鏈,來將函數內部的變量和方法傳遞到外面。

納尼,我們要突破作用域鏈?

什么是作用域鏈呀?

既然要說什么是作用域鏈,首先我們就得從 JS 中非常重要的一個概念說起。

這個概念就是對象。要知道,就想我現在的個性簽名一樣。

萬物皆虛 
萬事皆允

我們的 JS 中可也是號稱 “萬物皆是對象”的呀。

為什么這么說,其實在 JS 中,任何事物都可以通過一些特定的方法轉化成對象。

甚至一個標簽,我們也能將其轉化為對象,甚至可以直接添加方法。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">div</span>></span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">div</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> div = document.querySelector(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'div'</span>);
    div.a = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        alert(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'hello, 我是李鵬'</span>);
    }
    console.log(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typeof</span> div);
    console.log(div);
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

這里寫圖片描述 
可以看見,我們直接給 div 添加了一個新的屬性,叫做 a,而且這時候我們去打印我們的 a 的類型,你會發現,它直接就是一個 object,是不是感覺略屌?

既然說到我們萬物皆對象,那么創建對象總共有幾種方式呢?

1.1 對象的第一種創建方式

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 注意,這三種創建方式,是由系統提供的對象</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> str = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span>();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> num = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Number</span>();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> arr = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Array</span>();

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 而下面,則是一種自定義對象</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> obj = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>();
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

1.2 對象的第二種創建方式

需要注意,如果你看見成對的大括號,賦給一個變量,那么這個變量就一定是對象沒跑了。

而且,需要注意,在 { } 中書寫的內容都是用 , 連接,而且最后一個屬性一定不要加 ;,別問我是怎么知道的。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 創建對象</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> obj = {
        a:<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'lipeng'</span>,
        b:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>,
        c:<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>

        }
    };

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//給對象添加屬性</span>
    obj.d = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'hello'</span>;
    console.log(obj);
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul>

這里寫圖片描述

其實這種寫法還存在另外一種變種,那就是下面這種。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 創建對象</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> obj = {};
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//給對象添加屬性</span>
    obj.d = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'hello'</span>;
    console.log(obj);
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

這么寫的話,就相當於一個 new Object 了。

1.3 對象的第三種創建方式

我們除了上面的那兩種書寫方式,其實我們還可以去套用一下工廠模式,去直接創建一個對象。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">creatObject</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(sex,age)</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> a = {};
        a.sex = sex;
        a.age = age;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> a;
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> obj = creatObject(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'男'</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>);
    console.log(obj);
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

這里寫圖片描述

是不是跟其他語言中的便利構造器非常類似呢?

1.4 對象的第四種創建方式

其實我們還可以通過去構造一個函數,去直接創建一個對象。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">my</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.age = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'18'</span>;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.sex = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'nan'</span>;
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> obj = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> my();
    console.log(obj);
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

這里寫圖片描述

等等,發現貌似不太對,為什么這個類型直接是 my呢?

回頭看看上面的內容,你會發現,當我們打印這個內容的時候,實際上我們也會直接打印出這個類型具體是什么。

其實在上看到的 Object {age: "18", sex: "nan"},這種東西,在系統內容其實跟現在這種方法也是類似的。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">Object</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.XXX = XXX;
        .
        .
        .
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> obj = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>();
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

這里我補充一句,我們的 new 實際是開辟一個空間,那么我們根據誰去開辟空間呢?

我們根據的是函數去開辟空間,那么我們開辟了空間去給誰呢?

當然是給我們的 obj。

這時候我們就可以說,我們先開辟了一個空間給 a , 之后由開辟的這個空間去調動 my() 這個方法。

這非常類似於一個類的概念,但是實際上這其實是一個函數調用的過程。

而且需要注意,在這里添加屬性的時候,必須要去使用 this。

甚至我們可以更靈活一點,讓這些內容直接作為形參。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">my</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(sex,age)</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.sex = sex;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.age = age;
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> obj = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> my(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'nan'</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>);
    console.log(obj);
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

這里寫圖片描述

2.變量的作用域

我們剛才說了一大堆的對象,那剛才那些內容和作用域鏈有什么關系呢?

這是因為,我們的函數對象其實和其他對象一樣,都擁有可以通過代碼訪問的屬性,和一系列提供給 JS 引擎訪問的內部屬性。

而這些屬性當中,存在這個一個內部屬性 [[Scope]],因為很多個,放在一個類數組中,所以用兩個中括號包裹,而這個內部屬性就是 作用域

下面引用一段話。

Scope 該屬性內部包含了函數被創建的作用域中對象的集合, 
這個集合被稱之為 函數的作用域鏈, 
它決定了哪些數據能夠被函數訪問。

——–《ECMA-262 標准第三版》

既然我們想要繼續了解作用域鏈到底是什么東西,我們接下來就一起再來看看,什么是變量的作用域。

變量的作用域實際上就是“程序源代碼中定義這個變量的區域”,

其中全局變量有全局的作用域,它在 JS 的任何地方都是有定義的。

因為所有內容都籠罩在 Window 的光輝之下(笑)。

而函數內部聲明的變量只在函數體內有定義,他們只是一個局部的變量,作用域也只局限在函數內部。

要知道,函數參數也是局部變量,他們只在函數體內有定義。

而且在聲明全局變量的時候,可以不用去加 var,而聲明局部變量的時候,必須要加 var。

說到這里,簡單的說一下,函數的作用域。

2.1 函數的作用域

在一些類似於C 語言的編程語言中,花括號中每一段代碼都具有各自的作用域,而且變量在聲明他們的代碼片段以外是不可見的,我們稱之為 塊級作用域。

但是 JS 中沒有塊級作用域,取而代之的是函數作用域,變量在聲明他們的函數體以及這個函數體嵌套的任意函數體內都是有定義的,一定要注意,這里說的都是函數。 
這里寫圖片描述

而且在這里有一個非常有意思的東西,就是我們假如先不去聲明函數,直接調用,回事什么樣子的呢?

2.2 聲明提前(變量提升)

我們在說這些之前,先來看一段代碼。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> scope = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'lipeng'</span>;
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">aa</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        console.log(scope);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> scope = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"李鵬"</span>;
        console.log(scope);
    }
    aa();
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

這兩個log 分別會打印什么呢?

這里寫圖片描述

怎么樣,是不是完全沒想到?

這是因為,JS 函數中聲明的變量(但不涉及到賦值),在 JS 引擎預編譯的時候,都會將變量“提升”至函數的頂部,代碼的實際效果其實是這個樣子的。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> scope = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'lipeng'</span>;
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">aa</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> scope;
        console.log(scope);
        scope = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"李鵬"</span>;
        console.log(scope);
    }
    aa();
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

注意,函數外的 scope 完全是一個迷惑項,因為下面的內容從新聲明了一個 scope ,根據作用域的說明,它找到函數內部的內容就不會向它的上一級去進行尋找了,所以出現的才是 undefined

這里還體現出了 JS 編程時一個非常重要的原則,就是 “變量聲明全部放在函數體的頂部”。

2.3 屬性的變量

除了上面說的幾點之外,我們在日常的開發中還需要注意這么一件事。

當聲明一個 JS 全局變量的時候,實際上是定義了全局對象的一個屬性,

當使用 var 聲明一個變量的時候,創建的這個屬性實際實際上就是已經不可配置的了,

也就是說這個變量無法通過 Delete 運算符刪除。

但是如果是非嚴格模式的情況下,給一個沒有聲明的變量進行賦值的時候,

JS 會自動創建一個全局變量,通過這種方式創建的對象可以配置其屬性值,

並且還可以刪除他們。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 不可刪除的全局變量</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> scope_1 = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 全局對象的一個可刪除的屬性</span>
        scope_2 = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;

    console.log(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">delete</span> scope_1);
    console.log(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">delete</span> scope_2);
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

這里寫圖片描述

第一個刪除的時候,會直接返回 false,而第二個,它是作為一個全局對象的屬性,刪除的時候 會直接返回 true。

這是因為,全局變量是全局對象的屬性,這是在ECMAScript規范中強制規定的,

而對於局部變量則沒有如此規定,

但是我們可以想象,我們可以將局部變量當做是一個函數對象的屬性。

ECMAScript 稱該對象為“調用對象”,ECMAScript 中規范為 “聲明上下文對象”,

JS 中允許使用 this 關鍵字來引用全局對象,卻沒辦法引用局部變量中存放的對象,

這種存放局部變量的對象的特有性質,是一種對我們不可見的內部實現。

所以我們要去通過作用域鏈去配合使用一下。

但是從剛才賦值的地方就開始說 this,現在又再次說道了 this,那我們再簡單的來看一看,this 又是一個什么鬼?

3. this

在說 this 之前,首先先說一個東西,我們的全局變量全都是 window 的屬性。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> lipeng = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>;
    console.log(window);
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

這時候我們發現了一個問題。

這里寫圖片描述

我們剛才聲明的 lipeng,實際在 window 中是作為一個屬性放在里面的。

那假如我現在去聲明一個函數呢?

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">aa</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        console.log(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>);
    }
    aa();
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

這時候我們的函數中的 this ,又應該指向誰呢?

答案其實很簡單,直接聲明的函數是怎么聲明的?

那有是誰去調用的呢?

當然是我們的 window 咯。

這里寫圖片描述

那我其實去調用 aa() 的時候,其實也可以寫成 window.aa( );

那假如我現在換了一種寫法,

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">aa</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        getCall = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
            console.log(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>);
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>;
    }
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

這時候 this 又應該是誰呢?

在說 this 之前,先問大家一個問題,我這個函數內部的 getCall 應該怎么調用?

既然 getCall 是放在 aa() 內部的,那么你想調用 getCall ,就必須先調用 aa( )。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">aa</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        getCall = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
            console.log(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>);
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>;
    }
    aa();
    getCall();
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

除此之外,我們其實還可以直接通過點的方式去調用,即 aa().getCall();

這時候我們就應該能看見了,我們答應出來的 this 實際還是 window。

為什么呢?

這時因為,aa( ) 是 window 的,而這時候,我們的 getCall 實際也作為屬性賦值給了 window 。

我們將一個匿名函數給了 一個變量,而這個變量是全局變量,那你打印 this 不是window 還是 什么呀?

如果你覺得上面的內容你都明白了,那么再看這個呢?

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">style</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/css"</span>></span><span class="css" style="box-sizing: border-box;">
        <span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">div</span><span class="hljs-rules" style="box-sizing: border-box;">{
            <span class="hljs-rule" style="box-sizing: border-box;"><span class="hljs-attribute" style="box-sizing: border-box;">width</span>:<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 102, 102);"> <span class="hljs-number" style="box-sizing: border-box;">300</span>px</span></span>;
            <span class="hljs-rule" style="box-sizing: border-box;"><span class="hljs-attribute" style="box-sizing: border-box;">height</span>:<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 102, 102);"> <span class="hljs-number" style="box-sizing: border-box;">300</span>px</span></span>;
            <span class="hljs-rule" style="box-sizing: border-box;"><span class="hljs-attribute" style="box-sizing: border-box;">background</span>:<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 102, 102);"> red</span></span>;
        <span class="hljs-rule" style="box-sizing: border-box;">}</span></span>
    </span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">style</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">div</span>></span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">div</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> div = document.querySelector(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'div'</span>);
    div.onclick = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        console.log(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> timer = setInterval(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
            console.log(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>);
        },<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1000</span>);
    }
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li></ul>

這兩個 this 分別指了什么呢? 
這里寫圖片描述
第一個 this 指的 是 div , 因為我們是通過 div.onclick 去調用的。

而第二個 計時器,為什么打印的 window 呢?

這是因為,計時器實際上是一個 “函數回調”。

誰去對它進行回調呢? 自然是 window。

所以這才是打印 window 的原因。

4. 作用域鏈

當我們將剛才的內容都看懂了,明白我們可以將一個局部變量作為自定義對象的屬性的話,我們就可以從另外一個角度去解析變量作用域了。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">add</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(num1,num2)</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> sum = var1 + var2;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> sum;
    }
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

我們在 函數 add 被創建的時候,add 函數的作用域中就會被放進一個作用域鏈,而這個作用域鏈中又存放了一個全局對象,在這個全局對象中存放了當前所有的全局變量。

這里寫圖片描述

需要注意,當執行該函數的時候,會去創建一個叫做“運行期上下文”的內部對象,它定義了函數執行的時候,環境是什么樣子的。

每個運行期上下文都有自己的作用域鏈,用來進行標示符的解析,當運行上下文被創建的時候,它的作用域鏈被初始化為當前運行函數中的 “[[Scope]] 包含的對象” 。

這時候他們會直接按照在函數中出現的順序,被賦值到作用域鏈中,這時候他們又組成了一個新的對象,叫做 “活動對象”。

活動對象中包含了函數的所有局部變量,命名參數,參數集合,this等等。

這時候剛剛獲得的這個對象,就會被放在作用域鏈的最前端,方便之后的調用。

如果運行期上下文被銷毀,或者說函數執行完畢,那么里面的活動對象等也會隨之被銷毀。

這里使用一張別人的圖,可以很清楚明白的體現出來剛才所說的內容。

這里寫圖片描述

上面說的都是一些原理性的東西,那么我們接下來去說一些實質性的內容。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 全局作用域</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">A</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// A 的作用域,可以訪問到 a, b</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 但是訪問不到 c</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> b = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;
        <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">B</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// B 的作用域,可以訪問到 a, b, c</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> c = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>;
            console.log(a);
        }
        B();
    }
    A();

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 當 console.log(a); 的時候,</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// JS 引擎沿着 B 的作用域,A 的作用域,全局作用域的順序去進行查找</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 而這三個作用域實際上是存在循序的</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 我們將這種有序集合,叫做 作用域鏈</span>
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li></ul>

用文字性的東西來描述一下。

每一段 JS 代碼(全局代碼或者函數)都有一個與之關聯的作用域鏈,這個作用域鏈是一個對象列表或者鏈表,這組對象定義而這段代碼“作用域中”的變量。當 JS 需要查找變量x 的時候(這個過程叫做變量解析),它會從鏈中的第一個對象開始查找,如果這個對象有一個屬性名為 x 的屬性,則會直接使用這個屬性的值。如果第一個對象不存在名為 X 的屬性,JS 會繼續查找鏈上的下一個對象,以此類推,如果作用域上沒有任何一個對象含有屬性 x,那么就會認為這段代碼的作用域鏈上不存在 x,並最后爆出一個引用錯誤(ReferenceError)異常。

這時候我們應該已經明白了,作用域鏈是一個什么東西。

說白了,不就是一層嵌套一層么?

但是這個只是作用域鏈非常淺顯的一些東西,我們在下面的章節去說明。

現在的話,首先先來補充一下,在使用作用域鏈還存在一些注意事項。

4.1 作用域鏈的深度問題

從作用域鏈的結構可以看出,在運行期上下文的作用域鏈中,標識符所在的位置越深,讀寫速度就會越慢。

如上圖所示,因為全局變量總是存在於運行期上下文作用域鏈的最末端,因此在標識符解析的時候,查找全局變量是最慢的。

所以,在編寫代碼的時候應盡量少使用全局變量,盡可能使用局部變量。

一個好的經驗法則是:如果一個跨作用域的對象被引用了一次以上,則先把它存儲到局部變量里再使用。

這里寫圖片描述

4.2 改變作用域鏈

實際上運行上下文的過程中,還可以將作用域鏈臨時做一個更改,這時候其實可以用到兩種方法。

  1. with 語句
  2. try catch 語句

今天我就主要來說一下 with 語句。

with 語句的作用實際上就是某個對象添加到作用域鏈的頭部,然后執行內部的內容,當內容執行完畢之后,在將作用域鏈去恢復。

但是,實際上在嚴格模式(”use strict”;)中是禁止使用 with 語句的,而且在非嚴格模式下也是不推薦使用 with 語句的。

我們要盡可能的避免使用 with 語句,因為使用 with 的 JS 代碼非常難於優化,而且運行速度也要比正常代碼的運行速度更慢。

一般來說,我們只有在對象嵌套層級很深的情況下,才會去使用一下 with。

例如,我需要訪問 document.forms[0].people.lipeng 。

這時候每一次查詢,我都需要查詢非常多的內容,直到最后遍歷到 lipeng。

而如果我們明確我們要查找的內容具體在某個范圍的時候,我們可以這么做。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">with</span>(document.form[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]){
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 直接訪問表達那種的元素</span>
         people.value = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">""</span>;
         lipeng.value = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">""</span>;
    }
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

當然,如果我們不去使用with,直接使用代碼也是可以實現相同效果的。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> f = document.form[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>];
    f.people.value = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">""</span>;
    f.lipeng.value = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">""</span>;
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

記住一個問題,只有在查找標示符的時候才會用到作用域鏈,創建新的變量的時候不使用它。

5. 閉包

可是,有時候,我們卻不得不去訪問一些局部作用域內部的東西,比如兩個模塊函數,使用了相同的數據,

這里我們也只能把這些相同的數據放入全局變量,使得兩個函數模塊,都可以調用這些數據。

但是想想,如果這樣的需求很多,那么不久需要很多很多的全局變量,而濫用全局變量的不好之處,前面也說了,所以這並不是一種好的寫法。

這時候怎么去解決呢?

我們其實可以在一個函數內部,繼續定義函數,就像之前在函數A內部,定義了函數B,這樣我們只需要一個函數A的執行,就可以完成一整個邏輯。

內部的調用,都只能算是局部變量的調用,在全局只添加了一個函數A。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">lang</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"en"</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">head</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">meta</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">charset</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"UTF-8"</span>></span>
    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
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<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>

<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
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    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">A</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> arr = [];
        <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">a</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>};
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        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
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</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

這樣,我們本來需要三個全局變量的問題,就變成了只需要一個。當然,如何減少全局變量的方法是有很多種的,這里不做討論。

這里,我們就討論一種我們最常見的方法,也算是很常用的一種代碼書寫方法吧,它叫:閉包。

閉包實際上是 JS 語言特性的產物,因為 JS 采用的是 詞法作用域,

也就是說,函數的執行依賴於變量作用域,這個作用域是在函數定義時決定的,而不是函數調用時決定的。

為了實現這種詞法作用域,JS 函數對象的內部狀態不僅包含函數的代碼邏輯,還必須引用當前的作用域鏈。

函數對象可以通過作用域鏈相互關聯起來,函數體內部的變量都可以保存在函數作用域內,這種特性叫做閉包。

說的直白點,最常見的閉包就是在一個函數內創建另外一個函數,然后通過另外一個函數訪問這個函數的局部變量。

利用閉包我們可以突破作用域鏈,將函數內部的變量和方法傳遞到外部。

這也就是閉包。

這時候一起來看一個小栗子。

5.1 閉包的案例

因為這里涉及到很多的邏輯和操作,所以下面使用截圖來給大家演示。

這里寫圖片描述
這里寫圖片描述
這里寫圖片描述

這時候發現了一個問題,先引入的變量被后引入的變量直接覆蓋掉了。

我的天,這要是正常開發中,你同時非找你拼命不可。

根據我們之前的學習,其實可以定義兩個相同名稱的變量,而且不會相互影響,那就是函數。 
這里寫圖片描述

函數內的 1000 並不會影響函數外的 10,那我們現在就動了小心思。

我難道不能將兩個 JS 文件中的變量也用函數承裝起來么?

答案當然是可以的。

這里寫圖片描述

但是這樣的話,還會產生一個新的小問題,你既然定義一個函數,你就一定要去讓他執行一遍呀。

還記得之前說過,函數后面直接加 ( ),就是調用函數,所以這時候我們也可以,將我們的函數從新承裝一下。

這里寫圖片描述

注意,前面的函數也需要用括號包裹一下,否則會報錯的。

而這個看上去很奇怪的東西,它就是閉包。

5.2 閉包的三種寫法

閉包其實是有三種寫法的,上面看到的就是第一種。

<code class="hljs lisp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-list" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">function</span><span class="hljs-list" style="box-sizing: border-box;">()</span>{

})</span><span class="hljs-list" style="box-sizing: border-box;">()</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

實際上,我們還可以將 ( )挪進里面。

<code class="hljs lisp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-list" style="box-sizing: border-box;">(<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">function</span><span class="hljs-list" style="box-sizing: border-box;">()</span>{

}<span class="hljs-list" style="box-sizing: border-box;">()</span>)</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

除此之外,還存在這么一種寫法。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>

}();</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

這三種寫法實際上都是閉包,小伙伴們平常去查看一些第三方庫,經過會看到一些類似於上面的內容,如果到時候不明白,記得要回來查看一下這篇文章呦。

5.3 一些特殊的閉包

如果你覺得你明白了上的內容,那么可以試着去解釋一下下面這段代碼是什么意思吧。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
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    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> s = (<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> count = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> count++};
    })();
    console.log(s());
    console.log(s());
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>

粗略來看,這段代碼貌似是將函數賦值給一個 變量,

但是這段代碼實際上是定義了一段立即調用的函數,這個函數返回值賦值給變量。

而且除此之外,私有變量並不僅僅包含在一個單獨的閉包中,還可以讓同一個外部函數內的多個嵌套函數,一起訪問他,讓多個嵌套函數共享一個作用域鏈。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-doctype" style="color: rgb(102, 0, 102); box-sizing: border-box;"><!DOCTYPE html></span>
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    <span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>Document<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">title</span>></span>
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<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">type</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"text/javascript"</span>></span><span class="javascript" style="box-sizing: border-box;">
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">counter</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> n = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>{
            count:<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> n++},
            reset:<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> n=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>}
        }
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> c = counter(),d = counter();

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 他們之間是不會相互影響的</span>
    console.log(c.count());
    console.log(d.count());

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// reset 和 count 方法共享狀態</span>
    c.reset();

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 0 : 因為 C 被重置了</span>
    console.log(c.count());
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 1 : 因為 D 沒有被重置</span>
    console.log(d.count());
</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span>
<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">html</span>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul>

需要注意,每次調用 counter() 都會創建一個新的作用域鏈和一個新的私有變量。

兩個對象,彼此包含不同的私有變量,調用其中一個對象方法,不會影響到另外的一個對象。

6. 結語

這篇文章說到現在,已經算是結束了。

JS 是一門非常博大精深的語言,每一個人在這條路上都是苦行者,學有先后,希望所有的小伙伴,都能在這條路上越走越遠。

謹以此自勉。


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