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<!DOCTYPE html>
<html class="theme-next mist use-motion" lang="zh-Hans">
<head>
<meta charset="UTF-8"/>
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1"/>
<meta name="theme-color" content="#222">
<meta http-equiv="Cache-Control" content="no-transform" />
<meta http-equiv="Cache-Control" content="no-siteapp" />
<link href="/lib/fancybox/source/jquery.fancybox.css?v=2.1.5" rel="stylesheet" type="text/css" />
<link href="//fonts.googleapis.com/css?family=Lato:300,300italic,400,400italic,700,700italic&subset=latin,latin-ext" rel="stylesheet" type="text/css">
<link href="/lib/font-awesome/css/font-awesome.min.css?v=4.6.2" rel="stylesheet" type="text/css" />
<link href="/css/main.css?v=5.1.2" rel="stylesheet" type="text/css" />
<meta name="keywords" content="Hexo, NexT" />
<link rel="shortcut icon" type="image/x-icon" href="/favicon.ico?v=5.1.2" />
<meta name="description" content="责任始自梦中">
<meta property="og:type" content="website">
<meta property="og:title" content="numacraw">
<meta property="og:url" content="http://numacraw.com/index.html">
<meta property="og:site_name" content="numacraw">
<meta property="og:description" content="责任始自梦中">
<meta property="og:locale" content="zh-Hans">
<meta name="twitter:card" content="summary">
<meta name="twitter:title" content="numacraw">
<meta name="twitter:description" content="责任始自梦中">
<script type="text/javascript" id="hexo.configurations">
var NexT = window.NexT || {};
var CONFIG = {
root: '/',
scheme: 'Mist',
sidebar: {"position":"left","display":"always","offset":12,"offset_float":12,"b2t":false,"scrollpercent":false,"onmobile":false},
fancybox: true,
tabs: true,
motion: true,
duoshuo: {
userId: '0',
author: '博主'
},
algolia: {
applicationID: '',
apiKey: '',
indexName: '',
hits: {"per_page":10},
labels: {"input_placeholder":"Search for Posts","hits_empty":"We didn't find any results for the search: ${query}","hits_stats":"${hits} results found in ${time} ms"}
}
};
</script>
<link rel="canonical" href="http://numacraw.com/"/>
<title>numacraw</title>
</head>
<body itemscope itemtype="http://schema.org/WebPage" lang="zh-Hans">
<div class="container sidebar-position-left
page-home
">
<div class="headband"></div>
<header id="header" class="header" itemscope itemtype="http://schema.org/WPHeader">
<div class="header-inner"><div class="site-brand-wrapper">
<div class="site-meta ">
<div class="custom-logo-site-title">
<a href="/" class="brand" rel="start">
<span class="logo-line-before"><i></i></span>
<span class="site-title">numacraw</span>
<span class="logo-line-after"><i></i></span>
</a>
</div>
<p class="site-subtitle">嘿嘿嘿</p>
</div>
<div class="site-nav-toggle">
<button>
<span class="btn-bar"></span>
<span class="btn-bar"></span>
<span class="btn-bar"></span>
</button>
</div>
</div>
<nav class="site-nav">
<ul id="menu" class="menu">
<li class="menu-item menu-item-home">
<a href="/" rel="section">
<i class="menu-item-icon fa fa-fw fa-home"></i> <br />
首页
</a>
</li>
<li class="menu-item menu-item-archives">
<a href="/archives/" rel="section">
<i class="menu-item-icon fa fa-fw fa-archive"></i> <br />
归档
</a>
</li>
<li class="menu-item menu-item-tags">
<a href="/tags/" rel="section">
<i class="menu-item-icon fa fa-fw fa-tags"></i> <br />
标签
</a>
</li>
<li class="menu-item menu-item-commonweal">
<a href="/404/" rel="section">
<i class="menu-item-icon fa fa-fw fa-heartbeat"></i> <br />
公益404
</a>
</li>
</ul>
</nav>
</div>
</header>
<main id="main" class="main">
<div class="main-inner">
<div class="content-wrap">
<div id="content" class="content">
<section id="posts" class="posts-expand">
<article class="post post-type-normal" itemscope itemtype="http://schema.org/Article">
<div class="post-block">
<link itemprop="mainEntityOfPage" href="http://numacraw.com/2019/03/02/💅 背后的魔法/">
<span hidden itemprop="author" itemscope itemtype="http://schema.org/Person">
<meta itemprop="name" content="numacraw">
<meta itemprop="description" content="">
<meta itemprop="image" content="/images/avatar.jpg">
</span>
<span hidden itemprop="publisher" itemscope itemtype="http://schema.org/Organization">
<meta itemprop="name" content="numacraw">
</span>
<header class="post-header">
<h1 class="post-title" itemprop="name headline">
<a class="post-title-link" href="/2019/03/02/💅 背后的魔法/" itemprop="url">当我们使用<💅>时， 背后发生了啥</a></h1>
<div class="post-meta">
<span class="post-time">
<span class="post-meta-item-icon">
<i class="fa fa-calendar-o"></i>
</span>
<span class="post-meta-item-text">发表于</span>
<time title="创建于" itemprop="dateCreated datePublished" datetime="2019-03-02T18:46:52+08:00">
2019-03-02
</time>
</span>
<span id="/2019/03/02/💅 背后的魔法/" class="leancloud_visitors" data-flag-title="当我们使用<💅>时， 背后发生了啥">
<span class="post-meta-divider">|</span>
<span class="post-meta-item-icon">
<i class="fa fa-eye"></i>
</span>
<span class="post-meta-item-text">阅读次数 </span>
<span class="leancloud-visitors-count"></span>
</span>
</div>
</header>
<div class="post-body" itemprop="articleBody">
<p>#当我们使用<💅>时， 背后发生了啥</p>
<p>整理至一次部门的前端分享会，骚气 <💅> 背后的一些黑魔法。对于 Styled-Components 不作太多介绍，学习成本不高，看官方文档既可[<a href="https://www.styled-components.com/]。" target="_blank" rel="external">https://www.styled-components.com/]。</a></p>
<p>##一个 <💅> 组件长什么样</p>
<p>首先我们来看一个官网的🌰，简单但展示了怎么定义一个 styled-component 组件，并且如何使用。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">import</span> React <span class="keyword">from</span> <span class="string">"react"</span>;</div><div class="line"><span class="keyword">import</span> ReactDOM <span class="keyword">from</span> <span class="string">"react-dom"</span>;</div><div class="line"><span class="keyword">import</span> styled, { css } <span class="keyword">from</span> <span class="string">"styled-components"</span>;</div><div class="line"></div><div class="line"><span class="keyword">const</span> Button = styled.a<span class="string">`</span></div><div class="line"><span class="string"> display: inline-block;</span></div><div class="line"><span class="string"> border-radius: 3px;</span></div><div class="line"><span class="string"> padding: 0.5rem 0;</span></div><div class="line"><span class="string"> margin: 0.5rem 1rem;</span></div><div class="line"><span class="string"> width: 11rem;</span></div><div class="line"><span class="string"> background: transparent;</span></div><div class="line"><span class="string"> color: white;</span></div><div class="line"><span class="string"> border: 2px solid white;</span></div><div class="line"><span class="string"> <span class="subst">${props =></span></span></div><div class="line"><span class="string"><span class="subst"> props.primary &&</span></span></div><div class="line"><span class="string"><span class="subst"> css<span class="string">`</span></span></span></div><div class="line"><span class="string"><span class="subst"><span class="string"> background: white;</span></span></span></div><div class="line"><span class="string"><span class="subst"><span class="string"> color: palevioletred;</span></span></span></div><div class="line"><span class="string"><span class="subst"><span class="string"> `</span>}</span></span></div><div class="line"><span class="string">`</span>;</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">function</span> <span class="title">App</span>(<span class="params"></span>) </span>{</div><div class="line"> <span class="keyword">return</span> (</div><div class="line"> <div></div><div class="line"> <Button</div><div class="line"> href=<span class="string">"https://github.com/styled-components/styled-components"</span></div><div class="line"> target=<span class="string">"_blank"</span></div><div class="line"> rel=<span class="string">"noopener"</span></div><div class="line"> primary</div><div class="line"> ></div><div class="line"> GitHub</div><div class="line"> <<span class="regexp">/Button></span></div><div class="line"><span class="regexp"></span></div><div class="line"><span class="regexp"> <Button prefetch>Documentation</</span>Button></div><div class="line"> <<span class="regexp">/div></span></div><div class="line"><span class="regexp"> );</span></div><div class="line"><span class="regexp">}</span></div><div class="line"><span class="regexp"></span></div><div class="line"><span class="regexp">const rootElement = document.getElementById("root");</span></div><div class="line"><span class="regexp">ReactDOM.render(<App /</span>>, rootElement);</div></pre></td></tr></table></figure>
<ol>
<li><p>引入 styled-components</p>
</li>
<li><p>定义一个 styled-components 组件</p>
</li>
<li>消费一个 react 组件</li>
</ol>
<p>可以看到定义一个 styled-components 组件时候，用到了一个特别的东西 ——— <strong>模板字符串</strong>。</p>
<p>关于模板字符串可以看阮一峰的 ECMAScript 6 入门 [<a href="http://es6.ruanyifeng.com/#docs/string#模板字符串" target="_blank" rel="external">http://es6.ruanyifeng.com/#docs/string#模板字符串</a>]</p>
<p>##为什么是模板字符串</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">const</span> Button = styled.a<span class="string">`</span></div><div class="line"><span class="string"> display: inline-block;</span></div><div class="line"><span class="string"> border-radius: 3px;</span></div><div class="line"><span class="string"> padding: 0.5rem 0;</span></div><div class="line"><span class="string"> margin: 0.5rem 1rem;</span></div><div class="line"><span class="string"> width: 11rem;</span></div><div class="line"><span class="string"> background: transparent;</span></div><div class="line"><span class="string"> color: white;</span></div><div class="line"><span class="string"> border: 2px solid white;</span></div><div class="line"><span class="string"> <span class="subst">${props =></span></span></div><div class="line"><span class="string"><span class="subst"> props.primary &&</span></span></div><div class="line"><span class="string"><span class="subst"> css<span class="string">`</span></span></span></div><div class="line"><span class="string"><span class="subst"><span class="string"> background: white;</span></span></span></div><div class="line"><span class="string"><span class="subst"><span class="string"> color: palevioletred;</span></span></span></div><div class="line"><span class="string"><span class="subst"><span class="string"> `</span>}</span></span></div><div class="line"><span class="string">`</span>;</div></pre></td></tr></table></figure>
<p>虽然看起来怪怪的，但它是一个函数的调用。</p>
<p>接下来我们看看这个过程发生什么。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="comment">// 我们再简化一下官网的 🌰，把字符串模版里面的东西打印一哈</span></div><div class="line"><span class="built_in">console</span>.log<span class="string">` display: inline-block;<span class="subst">${()=><span class="string">'background: transparent;'</span>}</span> color: white;`</span></div><div class="line"><span class="comment">// ['display: inline-block;','color: white;'], ()=>'background: transparent;'</span></div></pre></td></tr></table></figure>
<p>打印出来的东西是一个数组，第一项是字符串数组，第二项是函数本身。我们看看字符模版作参数是什么样的规则，当插入变量时，字符串数组会有个分号隔开。</p>
<p>###开始实现一个 myStyled 函数</p>
<p>了解前面的一点知识我们可以实现我们自己的伪 Styled-Components。不过我们先实现一个调用方式看起来一样的函数。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">const</span> myStyled => <span class="function">(<span class="params">[style]</span>) =></span> styled;</div><div class="line"><span class="keyword">const</span> Button = myStyled<span class="string">`color: white`</span>;</div><div class="line"><span class="built_in">console</span>.log(Button)</div><div class="line"><span class="comment">// color: white;</span></div><div class="line"><span class="comment">// 不过它仅仅返回一个字符串，跟我们要实现的简陋版 Styled-Components 有点远</span></div></pre></td></tr></table></figure>
<p>###不传 props 版</p>
<p>就是一个生产 react 组件的函数，这里是简单的伪实现，真正的 styled-components 会把样式插在 <style> 内部样式表上</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">const</span> myStyled = <span class="function"><span class="params">TargetComponent</span> =></span> ([style]) =></div><div class="line"> <span class="class"><span class="keyword">class</span> <span class="keyword">extends</span> <span class="title">React</span>.<span class="title">Component</span> </span>{</div><div class="line"> componentDidMount() {</div><div class="line"> <span class="keyword">this</span>.element.setAttribute(<span class="string">"style"</span>, style);</div><div class="line"> }</div><div class="line"></div><div class="line"> render() {</div><div class="line"> <span class="keyword">return</span> (</div><div class="line"> <TargetComponent</div><div class="line"> {...this.props}</div><div class="line"> ref={element => (<span class="keyword">this</span>.element = element)}</div><div class="line"> /></div><div class="line"> );</div><div class="line"> }</div><div class="line"> };</div><div class="line"></div><div class="line"><span class="keyword">const</span> Button = myStyled(<span class="string">"button"</span>)<span class="string">`</span></div><div class="line"><span class="string"> display: inline-block;</span></div><div class="line"><span class="string"> border-radius: 3px;</span></div><div class="line"><span class="string"> padding: 0.5rem 0;</span></div><div class="line"><span class="string"> margin: 0.5rem 1rem;</span></div><div class="line"><span class="string"> width: 11rem;</span></div><div class="line"><span class="string"> background: transparent;</span></div><div class="line"><span class="string"> color: white;</span></div><div class="line"><span class="string"> border: 2px solid white;</span></div><div class="line"><span class="string">`</span>;</div></pre></td></tr></table></figure>
<p>###传 props 版</p>
<p>之前介绍，带函数的字符模版作参数会返回字符串数组，还有函数本身。我们用一个 reduce 对剩余参数进行处理，如果是函数就执行它。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div></pre></td><td class="code"><pre><div class="line"></div><div class="line"><span class="keyword">const</span> myStyled = <span class="function"><span class="params">TargetComponent</span> =></span> (strs, ...exprs) => <span class="class"><span class="keyword">class</span> <span class="keyword">extends</span> <span class="title">React</span>.<span class="title">Component</span> </span>{</div><div class="line"> init() {</div><div class="line"> <span class="keyword">const</span> style = exprs.reduce(<span class="function">(<span class="params">result, expr, index</span>) =></span> {</div><div class="line"> <span class="keyword">const</span> isFunc = <span class="keyword">typeof</span> expr === <span class="string">'function'</span>;</div><div class="line"> <span class="keyword">const</span> value = isFunc ? expr(<span class="keyword">this</span>.props) : expr;</div><div class="line"> </div><div class="line"> <span class="keyword">return</span> result + value + strs[index + <span class="number">1</span>];</div><div class="line"> }, strs[<span class="number">0</span>]);</div><div class="line"></div><div class="line"> <span class="keyword">this</span>.element.setAttribute(<span class="string">'style'</span>, style);</div><div class="line"> }</div><div class="line"></div><div class="line"> componentDidMount() {</div><div class="line"> <span class="keyword">this</span>.init();</div><div class="line"> }</div><div class="line"></div><div class="line"> componentDidUpdate() {</div><div class="line"> <span class="keyword">this</span>.init();</div><div class="line"> }</div><div class="line"></div><div class="line"> render() {</div><div class="line"> <span class="keyword">return</span> <TargetComponent {...this.props} ref={element => this.element = element } /></div><div class="line"> }</div><div class="line">};</div><div class="line"></div><div class="line">const Button = myStyled('button')`</div><div class="line"> display: inline-block;</div><div class="line"> border-radius: 3px;</div><div class="line"> padding: 0.5rem 0;</div><div class="line"> margin: 0.5rem 1rem;</div><div class="line"> width: 11rem;</div><div class="line"> background: transparent;</div><div class="line"> color: white;</div><div class="line"> border: 2px solid white;</div><div class="line"> ${props =></div><div class="line"> props.primary &&</div><div class="line"> css`</div><div class="line"> background: white;</div><div class="line"> color: palevioletred;</div><div class="line"> `}</div><div class="line">`;</div></pre></td></tr></table></figure>
<h3 id="回到问题的开始，为什么是字符串模板"><a href="#回到问题的开始，为什么是字符串模板" class="headerlink" title="回到问题的开始，为什么是字符串模板"></a>回到问题的开始，为什么是字符串模板</h3><p>我们可以看回刚刚的实现：</p>
<figure class="highlight plain"><figcaption><span>value = isFunc ? expr(this.props) : expr;```</span></figcaption><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div></pre></td><td class="code"><pre><div class="line"></div><div class="line">其实很简单，当遇到是函数时候，就把 props 传进去，这以便使用者可以用 props 来修改组件样式。所以，我们可以使用 <Button primary/> 来改变 Button 组件的样子。</div><div class="line"></div><div class="line">##唯一类名问题，用工具解决</div><div class="line"></div><div class="line">Styled-Components 的做法</div><div class="line"></div><div class="line">每使用一个 Styled-Components 组件，都会生成一个唯一的 className。在 react 渲染出来实际的 html ，如下所示：</div><div class="line"></div><div class="line">```html</div><div class="line"><style></div><div class="line">.dPbwXB {</div><div class="line"> font-size: 1.5em;</div><div class="line"> text-align: center;</div><div class="line"> color: palevioletred;</div><div class="line">}</div><div class="line"></style></div><div class="line"><h1 class="sc-gZMcBi dPbwXB">Hello World!</h1></div></pre></td></tr></table></figure>
<p>接下来我们让我看看实现的过程。</p>
<p>1.解析标签，生成 evaluatedStyles 字符</p>
<p>2.生成 css 类名，由 MurmurHash 算法生成。当引入库时，即有 counter 来计算唯一类名。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">const</span> counter = <span class="number">0</span>;</div><div class="line">counter++;</div><div class="line"><span class="keyword">const</span> componentId = <span class="string">'sc-'</span> + hash(<span class="string">'sc'</span> + counter);</div><div class="line"><span class="keyword">const</span> className = hash(componentId + evaluatedStyles);</div></pre></td></tr></table></figure>
<p>3.Css 预处理，用到一个库 Stylis.js。( <a href="https://github.com/thysultan/stylis.js">https://github.com/thysultan/stylis.js</a> )</p>
<p>4.将 CSS 字符串注入页面：现在应该 <style> 在组件的注释标记之后将 CSS 注入页面头部的元素中：</p>
<p>需要一提的是 <💅> 做出的优化</p>
<p>生成类名分为 静态类名 与 动态类名</p>
<p>例如，样式化组件 <Button /> 每次都会使用相同的静态类进行渲染。如果使用插值更改样式，例如<Button secondary />，则动态类将是不同的，而静态类将保持不变。</p>
<p>##使用 Styled-Components 需要注意的问题</p>
<ul>
<li><p>因为样式是插入在页面中 <style> 标签里面，属于内部样式，如果需要覆盖要用到！Important。</p>
</li>
<li><p>对于动态变化生成的类，并不会删除。原因很简单删除带来大的花销仅仅用做不保留，可以看看这个 issue 里的讨论。[<a href="https://github.com/styled-components/styled-components/issues/1431#issuecomment-358097912">https://github.com/styled-components/styled-components/issues/1431#issuecomment-358097912</a> ]</p>
</li>
<li><p>应在 render 函数外定义 styled components , 若在 render 函数里面定义的话，每次渲染过程会重新创建一个 styled-components 组件， 会大大降低渲染速度。 </p>
</li>
</ul>
</style></p>
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<h2 id="手写一个-AJAX"><a href="#手写一个-AJAX" class="headerlink" title="手写一个 AJAX"></a>手写一个 AJAX</h2><p>面试过程要求手写一个 AJAX ，于是乎水一篇博文。</p>
<pre><code>//创建 XMLHttpRequest 对象
var ajax = new XMLHttpRequest();
//规定请求的类型、URL 以及是否异步处理请求。
ajax.open('GET',url,true);
//发送信息至服务器时内容编码类型
ajax.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
//发送请求
ajax.send(null);
//接受服务器响应数据
ajax.onreadystatechange = function () {
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</code></pre>
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<p>##JavaScript 的数据类型<br>基本数据类型： Undefined,Null, Boolean, Number, String, and Symbol。<br>引用类型：Object。</p>
<p>typeof用于检测基本数据类型十分实用，其返回值是以下七种 undefined, boolean, number, string, object, function, symbol。</p>
<pre><code>typeof undefined //"undefined"
typeof '' //"string"
typeof 666 //"number"
typeof NaN //"number"
typeof true //"boolean"
typeof function(){} //"function"
typeof null //"object"
typeof [] //"object"
typeof {} //"object"
</code></pre><p>可以从上面代码看出，typeof 在检查 null, array, object时显得力不从心。对于 typeof null, 理应跟其他基本数据类型一样，返回”null”。然而并不是，这是 JavaScript 最初设计的意外。</p>
<p>##检测引用类型instanceof<br>instanceof 用于判断变量是否是某一个对象的实例。</p>
<pre><code>{} instanceof Object; // returns true
[] instanceof Array; // returns true
</code></pre><p>##可靠的 Object.prototype.toString()<br>可以通过toString() 来获取每个对象的类型。为了每个对象都能通过 Object.prototype.toString() 来检测，需要以 Function.prototype.call() 或者 Function.prototype.apply() 的形式来调用，传递要检查的对象作为第一个参数，称为thisArg。</p>
<pre><code>var toString = Object.prototype.toString;
toString.call(new Date); // [object Date]
toString.call(new String); // [object String]
toString.call(Math); // [object Math]
toString.call(undefined); // [object Undefined]
toString.call(null); // [object Null]
</code></pre>
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