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class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/article/%E6%B5%85%E6%9E%90defineProperty%E4%B8%8EProxy%E5%AE%9E%E7%8E%B0%E7%9A%84%E5%8F%8C%E5%90%91%E7%BB%91%E5%AE%9A.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>浅析defineProperty与Proxy实现的双向绑定</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5JavaScript%E6%95%B0%E6%8D%AE%E7%B1%BB%E5%9E%8B.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>深入JavaScript数据类型</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5Vue3%E6%BA%90%E7%A0%81%EF%BC%8C%E7%9C%8B%E7%9C%8BVue.use%E5%90%8E%E7%A9%B6%E7%AB%9F%E5%8F%91%E7%94%9F%E4%BA%86%E4%BB%80%E4%B9%88%EF%BC%9F.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>深入Vue3源码，看看Vue.use后究竟发生了什么？</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3Proxy%E4%B8%8EReflect.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>深入理解Proxy与Reflect</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3%E6%B5%8F%E8%A7%88%E5%99%A8%E7%BC%93%E5%AD%98%E6%9C%BA%E5%88%B6.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>深入理解浏览器缓存机制</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3%E6%B5%8F%E8%A7%88%E5%99%A8%E8%BF%90%E8%A1%8C%E5%8E%9F%E7%90%86.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>深入理解浏览器运行原理</p><!--]--></a><!----></div><!----></div><!--]--></div></section></div><div class="group" data-v-0e1b55d4><section class="VPSidebarItem level-0 collapsible collapsed" data-v-0e1b55d4 data-v-4a3e3544><div class="item" role="button" tabindex="0" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><h2 class="text" data-v-4a3e3544>学习笔记</h2><div class="caret" role="button" aria-label="toggle section" tabindex="0" data-v-4a3e3544><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" viewbox="0 0 24 24" class="caret-icon" data-v-4a3e3544><path d="M9,19c-0.3,0-0.5-0.1-0.7-0.3c-0.4-0.4-0.4-1,0-1.4l5.3-5.3L8.3,6.7c-0.4-0.4-0.4-1,0-1.4s1-0.4,1.4,0l6,6c0.4,0.4,0.4,1,0,1.4l-6,6C9.5,18.9,9.3,19,9,19z"></path></svg></div></div><div class="items" data-v-4a3e3544><!--[--><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/JavaScript.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>JavaScript基础</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/CSS.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>CSS基础</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/JavaScriptEnhanced.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>JavaScript进阶</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/Front-end%20Engineering.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>前端工程化</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/SSR.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>服务端渲染</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/React.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>React基础</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/React%20Hooks.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>React Hooks</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/Redux.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>Redux</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/React%20Router.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>React Router</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-4a3e3544 data-v-4a3e3544><div class="item" data-v-4a3e3544><div class="indicator" data-v-4a3e3544></div><a class="VPLink link link" href="/note/MySQL.html" data-v-4a3e3544><!--[--><p class="text" data-v-4a3e3544>MySQL</p><!--]--></a><!----></div><!----></div><!--]--></div></section></div><!--]--><!--[--><!--]--></nav></aside><div class="VPContent has-sidebar" id="VPContent" data-v-9d158fda data-v-585eef7d><div class="VPDoc has-sidebar has-aside" data-v-585eef7d data-v-a686a8a6><!--[--><!--]--><div class="container" data-v-a686a8a6><div class="aside" data-v-a686a8a6><div class="aside-curtain" data-v-a686a8a6></div><div class="aside-container" data-v-a686a8a6><div class="aside-content" data-v-a686a8a6><div class="VPDocAside" data-v-a686a8a6 data-v-31570b60><!--[--><!--]--><!--[--><!--]--><div class="VPDocAsideOutline" role="navigation" data-v-31570b60 data-v-ba8455b5><div class="content" data-v-ba8455b5><div class="outline-marker" data-v-ba8455b5></div><div class="outline-title" role="heading" aria-level="2" data-v-ba8455b5>On this page</div><nav aria-labelledby="doc-outline-aria-label" data-v-ba8455b5><span class="visually-hidden" id="doc-outline-aria-label" data-v-ba8455b5> Table of Contents for current page </span><ul class="root" data-v-ba8455b5 data-v-6713c9cc><!--[--><!--]--></ul></nav></div></div><!--[--><!--]--><div class="spacer" data-v-31570b60></div><!--[--><!--]--><!----><!--[--><!--]--><!--[--><!--]--></div></div></div></div><div class="content" data-v-a686a8a6><div class="content-container" data-v-a686a8a6><!--[--><!--]--><!----><main class="main" data-v-a686a8a6><div style="position:relative;" class="vp-doc _article_%E6%B7%B1%E5%85%A5JavaScript%E6%95%B0%E6%8D%AE%E7%B1%BB%E5%9E%8B" data-v-a686a8a6><div><h2 id="深入javascript数据类型" tabindex="-1">深入JavaScript数据类型 <a class="header-anchor" href="#深入javascript数据类型" aria-label="Permalink to "深入JavaScript数据类型""></a></h2><p>JavaScript包含以下几种数据类型：</p><ul><li>Number 数字</li><li>String 字符串</li><li>Boolean 布尔值</li><li>Symbol 符号 （ES6新增）</li><li>Object 对象 <ul><li>Function 函数</li><li>Array 数组</li><li>Date 日期</li><li>RegExp 正则表达式</li><li>...</li></ul></li><li>null 空</li><li>undefined 未定义</li></ul><p>从语言底层值的可变与不可变，可以将JS中的数据分为两种：不可变值（原始类型）和可变值（引用类型）</p><p>除了Object及继承自Object的特殊对象，其他的类型都为<strong>原始类型</strong>。</p><h2 id="typeof运算符" tabindex="-1">typeof运算符 <a class="header-anchor" href="#typeof运算符" aria-label="Permalink to "typeof运算符""></a></h2><p>除了null，所有<strong>原始类型</strong>都可以通过<code>typeof</code>运算符得到不同的结果</p><p>而null与object通过<code>typeof</code>运算符得到的结果都为<code>'object'</code></p><div class="language-javascript vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki github-dark vp-code-dark"><code><span class="line"><span style="color:#6A737D;">// 除了 null 其他原始类型的变量都可以通过 typeof 得到其类型</span></span>
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<span class="line"><span style="color:#6A737D;">// 而 null 与 object 通过 typeof 运算得到的都是 'object'</span></span>
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<span class="line"><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">targets</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> [</span><span style="color:#79B8FF;">18</span><span style="color:#E1E4E8;">, </span><span style="color:#9ECBFF;">'Ziu'</span><span style="color:#E1E4E8;">, </span><span style="color:#79B8FF;">true</span><span style="color:#E1E4E8;">, </span><span style="color:#B392F0;">Symbol</span><span style="color:#E1E4E8;">(</span><span style="color:#9ECBFF;">''</span><span style="color:#E1E4E8;">), {}, </span><span style="color:#79B8FF;">null</span><span style="color:#E1E4E8;">, </span><span style="color:#79B8FF;">undefined</span><span style="color:#E1E4E8;">]</span></span>
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<span class="line"></span>
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<span class="line"><span style="color:#F97583;">for</span><span style="color:#E1E4E8;"> (</span><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">t</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">of</span><span style="color:#E1E4E8;"> targets) {</span></span>
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<span class="line"><span style="color:#E1E4E8;"> console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(</span><span style="color:#F97583;">typeof</span><span style="color:#E1E4E8;"> t) </span><span style="color:#6A737D;">// number string boolean symbol object object undefined</span></span>
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<span class="line"><span style="color:#E1E4E8;">}</span></span></code></pre><pre class="shiki github-light vp-code-light"><code><span class="line"><span style="color:#6A737D;">// 除了 null 其他原始类型的变量都可以通过 typeof 得到其类型</span></span>
|
||
<span class="line"><span style="color:#6A737D;">// 而 null 与 object 通过 typeof 运算得到的都是 'object'</span></span>
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<span class="line"><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">targets</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> [</span><span style="color:#005CC5;">18</span><span style="color:#24292E;">, </span><span style="color:#032F62;">'Ziu'</span><span style="color:#24292E;">, </span><span style="color:#005CC5;">true</span><span style="color:#24292E;">, </span><span style="color:#6F42C1;">Symbol</span><span style="color:#24292E;">(</span><span style="color:#032F62;">''</span><span style="color:#24292E;">), {}, </span><span style="color:#005CC5;">null</span><span style="color:#24292E;">, </span><span style="color:#005CC5;">undefined</span><span style="color:#24292E;">]</span></span>
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<span class="line"></span>
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||
<span class="line"><span style="color:#D73A49;">for</span><span style="color:#24292E;"> (</span><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">t</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">of</span><span style="color:#24292E;"> targets) {</span></span>
|
||
<span class="line"><span style="color:#24292E;"> console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(</span><span style="color:#D73A49;">typeof</span><span style="color:#24292E;"> t) </span><span style="color:#6A737D;">// number string boolean symbol object object undefined</span></span>
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||
<span class="line"><span style="color:#24292E;">}</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h3 id="typeof-null-object" tabindex="-1"><code>typeof null === 'object'</code> <a class="header-anchor" href="#typeof-null-object" aria-label="Permalink to "`typeof null === 'object'`""></a></h3><p>这里援引MDN的解释：</p><blockquote><p>在 JavaScript 最初的实现中，JavaScript 中的值是由一个表示类型的标签和实际数据值表示的。对象的类型标签是 0。</p><p>由于 null 代表的是空指针（大多数平台下值为 0x00），因此，null 的类型标签是 0，typeof null 也因此返回 "object"。</p></blockquote><h3 id="关于new操作符" tabindex="-1">关于new操作符 <a class="header-anchor" href="#关于new操作符" aria-label="Permalink to "关于new操作符""></a></h3><p>所有使用 new 操作符调用的构造函数，都将返回非基本类型（object 或 function）</p><ul><li>大多数返回的是一个对象，即 <code>object</code></li><li>而构造函数Function返回的是 <code>function</code></li></ul><div class="language-javascript vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki github-dark vp-code-dark"><code><span class="line"><span style="color:#6A737D;">// 针对普通构造函数</span></span>
|
||
<span class="line"><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">str</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">new</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">String</span><span style="color:#E1E4E8;">(</span><span style="color:#9ECBFF;">'Ziu'</span><span style="color:#E1E4E8;">)</span></span>
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<span class="line"><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">num</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">new</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">Number</span><span style="color:#E1E4E8;">(</span><span style="color:#79B8FF;">100</span><span style="color:#E1E4E8;">)</span></span>
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||
<span class="line"></span>
|
||
<span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(</span><span style="color:#F97583;">typeof</span><span style="color:#E1E4E8;"> str) </span><span style="color:#6A737D;">// 'object'</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(</span><span style="color:#F97583;">typeof</span><span style="color:#E1E4E8;"> num) </span><span style="color:#6A737D;">// 'object'</span></span>
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||
<span class="line"></span>
|
||
<span class="line"><span style="color:#6A737D;">// 针对 Function 构造函数</span></span>
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||
<span class="line"><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">fun</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">new</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">Function</span><span style="color:#E1E4E8;">()</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(</span><span style="color:#F97583;">typeof</span><span style="color:#E1E4E8;"> fun) </span><span style="color:#6A737D;">// 'function'</span></span></code></pre><pre class="shiki github-light vp-code-light"><code><span class="line"><span style="color:#6A737D;">// 针对普通构造函数</span></span>
|
||
<span class="line"><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">str</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">new</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">String</span><span style="color:#24292E;">(</span><span style="color:#032F62;">'Ziu'</span><span style="color:#24292E;">)</span></span>
|
||
<span class="line"><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">num</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">new</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">Number</span><span style="color:#24292E;">(</span><span style="color:#005CC5;">100</span><span style="color:#24292E;">)</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(</span><span style="color:#D73A49;">typeof</span><span style="color:#24292E;"> str) </span><span style="color:#6A737D;">// 'object'</span></span>
|
||
<span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(</span><span style="color:#D73A49;">typeof</span><span style="color:#24292E;"> num) </span><span style="color:#6A737D;">// 'object'</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="color:#6A737D;">// 针对 Function 构造函数</span></span>
|
||
<span class="line"><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">fun</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">new</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">Function</span><span style="color:#24292E;">()</span></span>
|
||
<span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(</span><span style="color:#D73A49;">typeof</span><span style="color:#24292E;"> fun) </span><span style="color:#6A737D;">// 'function'</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><h3 id="字符串原始值和字符串对象" tabindex="-1">字符串原始值和字符串对象 <a class="header-anchor" href="#字符串原始值和字符串对象" aria-label="Permalink to "字符串原始值和字符串对象""></a></h3><p>typeof 操作符区分 <code>String</code> 对象和原始字符串值：</p><div class="language-javascript vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki github-dark vp-code-dark"><code><span class="line"><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">s1</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> </span><span style="color:#9ECBFF;">''</span></span>
|
||
<span class="line"><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">s2</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">new</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">String</span><span style="color:#E1E4E8;">(</span><span style="color:#9ECBFF;">''</span><span style="color:#E1E4E8;">)</span></span>
|
||
<span class="line"><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">s3</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">String</span><span style="color:#E1E4E8;">(</span><span style="color:#9ECBFF;">''</span><span style="color:#E1E4E8;">)</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(s1 </span><span style="color:#F97583;">instanceof</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">String</span><span style="color:#E1E4E8;">) </span><span style="color:#6A737D;">// false</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(s2 </span><span style="color:#F97583;">instanceof</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">String</span><span style="color:#E1E4E8;">) </span><span style="color:#6A737D;">// true</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(s3 </span><span style="color:#F97583;">instanceof</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">String</span><span style="color:#E1E4E8;">) </span><span style="color:#6A737D;">// false</span></span></code></pre><pre class="shiki github-light vp-code-light"><code><span class="line"><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">s1</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> </span><span style="color:#032F62;">''</span></span>
|
||
<span class="line"><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">s2</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">new</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">String</span><span style="color:#24292E;">(</span><span style="color:#032F62;">''</span><span style="color:#24292E;">)</span></span>
|
||
<span class="line"><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">s3</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">String</span><span style="color:#24292E;">(</span><span style="color:#032F62;">''</span><span style="color:#24292E;">)</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(s1 </span><span style="color:#D73A49;">instanceof</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">String</span><span style="color:#24292E;">) </span><span style="color:#6A737D;">// false</span></span>
|
||
<span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(s2 </span><span style="color:#D73A49;">instanceof</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">String</span><span style="color:#24292E;">) </span><span style="color:#6A737D;">// true</span></span>
|
||
<span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(s3 </span><span style="color:#D73A49;">instanceof</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">String</span><span style="color:#24292E;">) </span><span style="color:#6A737D;">// false</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>通过new操作符创建的是一个对象，它将被添加到原型链上（详见new一个对象时发生了什么）</p><p>而直接调用 String 函数，返回的是一个字符串原始值，本质上 <code>s3</code> 和 <code>s1</code> 是相同的</p><p>而<code>s1</code>变量自创建之初，就是原始类型，没有挂载到原型链上，自然也就无法通过<code>instanceof</code>检查一个对象</p><p>而之所以能够在<code>s1</code>上调用字符串的方法，是因为<strong>包装类型</strong>（见下文）</p><blockquote><p>instanceof 运算符用于检测构造函数的 prototype 属性是否出现在某个实例对象的原型链上。</p><p>在本例中，instanceof用于检查s1变量的原型链上，是否包含构造函数String的显式原型（String.prototype），即：s1是否由String创建</p></blockquote><h2 id="包装类型" tabindex="-1">包装类型 <a class="header-anchor" href="#包装类型" aria-label="Permalink to "包装类型""></a></h2><p>除了null和undefined，所有<strong>原始类型</strong>都有其相应的<strong>对象包装类型</strong>，例如<code>18</code>的对象包装类型是<code>Number</code>，而<code>'Ziu'</code>的对象包装类是<code>String</code></p><p>这为处理原始值提供可用的方法，例如，<code>Number</code>对象提供了<code>toFixed()</code>这样的方法。</p><p>当我们在原始值上访问属性时，JavaScript会<strong>自动将值包装到相应的包装对象中</strong>，并访问对象上的属性：</p><div class="language-javascript vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki github-dark vp-code-dark"><code><span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">((</span><span style="color:#79B8FF;">18.8</span><span style="color:#E1E4E8;">).</span><span style="color:#B392F0;">toFixed</span><span style="color:#E1E4E8;">()) </span><span style="color:#6A737D;">// '19'</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(</span><span style="color:#9ECBFF;">'Ziu'</span><span style="color:#E1E4E8;">.</span><span style="color:#B392F0;">toUpperCase</span><span style="color:#E1E4E8;">()) </span><span style="color:#6A737D;">// 'ZIU'</span></span></code></pre><pre class="shiki github-light vp-code-light"><code><span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">((</span><span style="color:#005CC5;">18.8</span><span style="color:#24292E;">).</span><span style="color:#6F42C1;">toFixed</span><span style="color:#24292E;">()) </span><span style="color:#6A737D;">// '19'</span></span>
|
||
<span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(</span><span style="color:#032F62;">'Ziu'</span><span style="color:#24292E;">.</span><span style="color:#6F42C1;">toUpperCase</span><span style="color:#24292E;">()) </span><span style="color:#6A737D;">// 'ZIU'</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="使用tostring检查对象类型" tabindex="-1">使用toString检查对象类型 <a class="header-anchor" href="#使用tostring检查对象类型" aria-label="Permalink to "使用toString检查对象类型""></a></h2><p>由Object派生的每个特殊对象类型都有 <code>toString</code> 方法，而且他们都被不同程度的改写：</p><div class="language-javascript vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki github-dark vp-code-dark"><code><span class="line"><span style="color:#6A737D;">// 注意，不应当在null或undefined上调用任何方法，这将抛出错误，因为他们没有对应任何包装类型</span></span>
|
||
<span class="line"><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">targets</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> [</span><span style="color:#79B8FF;">18</span><span style="color:#E1E4E8;">, </span><span style="color:#9ECBFF;">'Ziu'</span><span style="color:#E1E4E8;">, </span><span style="color:#79B8FF;">true</span><span style="color:#E1E4E8;">, </span><span style="color:#B392F0;">Symbol</span><span style="color:#E1E4E8;">(</span><span style="color:#9ECBFF;">''</span><span style="color:#E1E4E8;">), {}, </span><span style="color:#F97583;">new</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">Date</span><span style="color:#E1E4E8;">()]</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="color:#F97583;">for</span><span style="color:#E1E4E8;"> (</span><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">t</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">of</span><span style="color:#E1E4E8;"> targets) {</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;"> console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(t.</span><span style="color:#B392F0;">toString</span><span style="color:#E1E4E8;">()) </span><span style="color:#6A737D;">// '18' 'Ziu' 'true' 'Symbol()' '[object Object]' Sat Feb 18 2023 20:49:31 GMT+0800 (GMT+08:00)</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;">}</span></span></code></pre><pre class="shiki github-light vp-code-light"><code><span class="line"><span style="color:#6A737D;">// 注意，不应当在null或undefined上调用任何方法，这将抛出错误，因为他们没有对应任何包装类型</span></span>
|
||
<span class="line"><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">targets</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> [</span><span style="color:#005CC5;">18</span><span style="color:#24292E;">, </span><span style="color:#032F62;">'Ziu'</span><span style="color:#24292E;">, </span><span style="color:#005CC5;">true</span><span style="color:#24292E;">, </span><span style="color:#6F42C1;">Symbol</span><span style="color:#24292E;">(</span><span style="color:#032F62;">''</span><span style="color:#24292E;">), {}, </span><span style="color:#D73A49;">new</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">Date</span><span style="color:#24292E;">()]</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="color:#D73A49;">for</span><span style="color:#24292E;"> (</span><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">t</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">of</span><span style="color:#24292E;"> targets) {</span></span>
|
||
<span class="line"><span style="color:#24292E;"> console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(t.</span><span style="color:#6F42C1;">toString</span><span style="color:#24292E;">()) </span><span style="color:#6A737D;">// '18' 'Ziu' 'true' 'Symbol()' '[object Object]' Sat Feb 18 2023 20:49:31 GMT+0800 (GMT+08:00)</span></span>
|
||
<span class="line"><span style="color:#24292E;">}</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>这个 <code>toString</code> 方法定义在 Object.prototype 上，被其他特殊对象类型所继承。</p><ul><li>如果我们直接调用 <code>Object.prototype.toString()</code> 那么得到的是 <code>[object Object]</code> 因为这是在Object上调用得到的结果</li><li>如果我们在各自不同的对象上调用 <code>toString</code> ，得到的是不同对象改写后的结果</li></ul><p>因为Object是所有子类的父类，所以任何类型的对象都可以通过this绑定的方式，调用<code>Object.prototype.toString()</code>方法，返回该对象的类型的字符串表示</p><p>这也是大多数情况下判断对象类型的方法：</p><div class="language-js vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">js</span><pre class="shiki github-dark vp-code-dark"><code><span class="line"><span style="color:#F97583;">function</span><span style="color:#E1E4E8;"> </span><span style="color:#B392F0;">classof</span><span style="color:#E1E4E8;">(</span><span style="color:#FFAB70;">target</span><span style="color:#E1E4E8;">) {</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">const</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">res</span><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">=</span><span style="color:#E1E4E8;"> </span><span style="color:#79B8FF;">Object</span><span style="color:#E1E4E8;">.</span><span style="color:#79B8FF;">prototype</span><span style="color:#E1E4E8;">.toString.</span><span style="color:#B392F0;">call</span><span style="color:#E1E4E8;">(target) </span><span style="color:#6A737D;">// [object xxx]</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;"> </span><span style="color:#F97583;">return</span><span style="color:#E1E4E8;"> res.</span><span style="color:#B392F0;">slice</span><span style="color:#E1E4E8;">(</span><span style="color:#79B8FF;">8</span><span style="color:#E1E4E8;">, </span><span style="color:#F97583;">-</span><span style="color:#79B8FF;">1</span><span style="color:#E1E4E8;">)</span></span>
|
||
<span class="line"><span style="color:#E1E4E8;">}</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="color:#E1E4E8;">console.</span><span style="color:#B392F0;">log</span><span style="color:#E1E4E8;">(</span><span style="color:#B392F0;">classof</span><span style="color:#E1E4E8;">({})) </span><span style="color:#6A737D;">// Object</span></span></code></pre><pre class="shiki github-light vp-code-light"><code><span class="line"><span style="color:#D73A49;">function</span><span style="color:#24292E;"> </span><span style="color:#6F42C1;">classof</span><span style="color:#24292E;">(</span><span style="color:#E36209;">target</span><span style="color:#24292E;">) {</span></span>
|
||
<span class="line"><span style="color:#24292E;"> </span><span style="color:#D73A49;">const</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">res</span><span style="color:#24292E;"> </span><span style="color:#D73A49;">=</span><span style="color:#24292E;"> </span><span style="color:#005CC5;">Object</span><span style="color:#24292E;">.</span><span style="color:#005CC5;">prototype</span><span style="color:#24292E;">.toString.</span><span style="color:#6F42C1;">call</span><span style="color:#24292E;">(target) </span><span style="color:#6A737D;">// [object xxx]</span></span>
|
||
<span class="line"><span style="color:#24292E;"> </span><span style="color:#D73A49;">return</span><span style="color:#24292E;"> res.</span><span style="color:#6F42C1;">slice</span><span style="color:#24292E;">(</span><span style="color:#005CC5;">8</span><span style="color:#24292E;">, </span><span style="color:#D73A49;">-</span><span style="color:#005CC5;">1</span><span style="color:#24292E;">)</span></span>
|
||
<span class="line"><span style="color:#24292E;">}</span></span>
|
||
<span class="line"></span>
|
||
<span class="line"><span style="color:#24292E;">console.</span><span style="color:#6F42C1;">log</span><span style="color:#24292E;">(</span><span style="color:#6F42C1;">classof</span><span style="color:#24292E;">({})) </span><span style="color:#6A737D;">// Object</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><h3 id="核心原理" tabindex="-1">核心原理 <a class="header-anchor" href="#核心原理" aria-label="Permalink to "核心原理""></a></h3><p><a href="https://juejin.cn/post/6844903477940846600" target="_blank" rel="noreferrer">深入Object.prototype.toString</a></p><p>参考ECMA6规范文档：<a href="https://tc39.es/ecma262/multipage/fundamental-objects.html#sec-object.prototype.tostring" target="_blank" rel="noreferrer">Object.prototype.toString()</a></p><h4 id="es5标准下-object-prototype-tostring-执行原理" tabindex="-1">ES5标准下 <code>Object.prototype.toString</code> 执行原理 <a class="header-anchor" href="#es5标准下-object-prototype-tostring-执行原理" aria-label="Permalink to "ES5标准下 `Object.prototype.toString` 执行原理""></a></h4><ul><li>如果 <code>this</code> 是 <code>undefined</code><ul><li>返回 <code>[object Undefined]</code></li></ul></li><li>如果 <code>this</code> 是 <code>null </code><ul><li>返回 <code>[object Null]</code></li></ul></li><li>令 <code>O</code> 为以 <code>this</code> 作为参数调用 <code>ToObject</code> 的结果</li><li>令 <code>class</code> 为 <code>O</code> 的内部属性 <code>[[Class]]</code> 的值</li><li>返回三个字符串 <code>"[object"</code> <code>class</code> 以及 <code>"]"</code> 拼接而成的字符串。</li></ul><h4 id="es6标准下-object-prototype-tostring-执行原理" tabindex="-1">ES6标准下 <code>Object.prototype.toString</code> 执行原理 <a class="header-anchor" href="#es6标准下-object-prototype-tostring-执行原理" aria-label="Permalink to "ES6标准下 `Object.prototype.toString` 执行原理""></a></h4><p><code>Object.prototype.toString()</code>被调用时，会进行如下步骤：</p><p><img src="/assets/Object.prototype.toString.70e8308c.png" alt="Object.prototype.toString()"></p><p>在ES6里，之前的内部属性 <code>[[Class]]</code> 不再使用，取而代之的是一系列的 internal slot</p></div></div></main><footer class="VPDocFooter" data-v-a686a8a6 data-v-0b8e2cba><!--[--><!--]--><div class="edit-info" data-v-0b8e2cba><div class="edit-link" data-v-0b8e2cba><a class="VPLink link vp-external-link-icon no-icon edit-link-button" href="https://github.com/ZiuChen/ZiuChen.github.io/edit/main/docs/article/深入JavaScript数据类型.md" target="_blank" rel="noreferrer" data-v-0b8e2cba><!--[--><svg xmlns="http://www.w3.org/2000/svg" viewbox="0 0 24 24" class="edit-link-icon" aria-label="edit icon" data-v-0b8e2cba><path d="M18,23H4c-1.7,0-3-1.3-3-3V6c0-1.7,1.3-3,3-3h7c0.6,0,1,0.4,1,1s-0.4,1-1,1H4C3.4,5,3,5.4,3,6v14c0,0.6,0.4,1,1,1h14c0.6,0,1-0.4,1-1v-7c0-0.6,0.4-1,1-1s1,0.4,1,1v7C21,21.7,19.7,23,18,23z"></path><path d="M8,17c-0.3,0-0.5-0.1-0.7-0.3C7,16.5,6.9,16.1,7,15.8l1-4c0-0.2,0.1-0.3,0.3-0.5l9.5-9.5c1.2-1.2,3.2-1.2,4.4,0c1.2,1.2,1.2,3.2,0,4.4l-9.5,9.5c-0.1,0.1-0.3,0.2-0.5,0.3l-4,1C8.2,17,8.1,17,8,17zM9.9,12.5l-0.5,2.1l2.1-0.5l9.3-9.3c0.4-0.4,0.4-1.1,0-1.6c-0.4-0.4-1.2-0.4-1.6,0l0,0L9.9,12.5z M18.5,2.5L18.5,2.5L18.5,2.5z"></path></svg> Edit this page on GitHub<!--]--></a></div><div class="last-updated" data-v-0b8e2cba><p class="VPLastUpdated" data-v-0b8e2cba data-v-a6f11a31>Updated Date: <time datetime="2024-01-06T06:57:39.000Z" data-v-a6f11a31></time></p></div></div><nav class="prev-next" data-v-0b8e2cba><div class="pager" data-v-0b8e2cba><a class="pager-link prev" href="/article/%E6%B5%85%E6%9E%90defineProperty%E4%B8%8EProxy%E5%AE%9E%E7%8E%B0%E7%9A%84%E5%8F%8C%E5%90%91%E7%BB%91%E5%AE%9A.html" data-v-0b8e2cba><span class="desc" data-v-0b8e2cba>Previous page</span><span class="title" data-v-0b8e2cba>浅析defineProperty与Proxy实现的双向绑定</span></a></div><div class="pager" data-v-0b8e2cba><a class="pager-link next" href="/article/%E6%B7%B1%E5%85%A5Vue3%E6%BA%90%E7%A0%81%EF%BC%8C%E7%9C%8B%E7%9C%8BVue.use%E5%90%8E%E7%A9%B6%E7%AB%9F%E5%8F%91%E7%94%9F%E4%BA%86%E4%BB%80%E4%B9%88%EF%BC%9F.html" data-v-0b8e2cba><span class="desc" data-v-0b8e2cba>Next page</span><span class="title" data-v-0b8e2cba>深入Vue3源码，看看Vue.use后究竟发生了什么？</span></a></div></nav></footer><!--[--><!--]--></div></div></div><!--[--><!--]--></div></div><footer class="VPFooter has-sidebar" data-v-9d158fda data-v-2c80950a><div class="container" data-v-2c80950a><p class="message" data-v-2c80950a>Released under the MIT License.</p><p class="copyright" data-v-2c80950a>Copyright © 2019-present Ziu Chen</p></div></footer><!--[--><!--]--></div></div>
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