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level3"><a href="/blog/pages/f344d070a1031ef7/#babel-register-模块" class="sidebar-link">@babel/register 模块</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f344d070a1031ef7/#babel-api" class="sidebar-link">babel API</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f344d070a1031ef7/#babel-polyfill" class="sidebar-link">@babel/polyfill</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f344d070a1031ef7/#浏览器环境" class="sidebar-link">浏览器环境</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/f344d070a1031ef7/#traceur-转码器" class="sidebar-link">Traceur 转码器</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/f344d070a1031ef7/#直接插入网页" class="sidebar-link">直接插入网页</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f344d070a1031ef7/#在线转换" class="sidebar-link">在线转换</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/f344d070a1031ef7/#命令行转换" 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href="/blog/pages/02c86eb2792f3262/" class="sidebar-link">Symbol</a></li><li><a href="/blog/pages/0c21dae358fca16b/" class="sidebar-link">Set 和 Map 数据结构</a></li><li><a href="/blog/pages/f56ec2ab97d60483/" class="sidebar-link">Proxy</a></li><li><a href="/blog/pages/74de3e45e4491e95/" class="sidebar-link">Reflect</a></li><li><a href="/blog/pages/2810ae8985e9bd52/" class="sidebar-link">Promise 对象</a></li><li><a href="/blog/pages/48df907ad3570f3d/" class="sidebar-link">Iterator 和 for-of 循环</a></li><li><a href="/blog/pages/718b48ed9ce0adce/" class="sidebar-link">Generator 函数的语法</a></li><li><a href="/blog/pages/75af7031eb66847b/" class="sidebar-link">Generator 函数的异步应用</a></li><li><a href="/blog/pages/3777253e65bac487/" class="sidebar-link">async 函数</a></li><li><a href="/blog/pages/e831e1593c82bbe0/" class="sidebar-link">Class 的基本语法</a></li><li><a href="/blog/pages/83f8c3a0cd87dd83/" class="sidebar-link">Class 的继承</a></li><li><a href="/blog/pages/efe2fb04eb8ac5fb/" class="sidebar-link">Module 的语法</a></li><li><a href="/blog/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/blog/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/blog/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/blog/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/blog/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06970110>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-01-12</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">ECMAScript 6 简介<!----></h1>  <div class="theme-vdoing-content content__default"><blockquote><p>说明：本章内容为博主在原教程基础上添加自己的学习笔记，来源<a href="http://es6.ruanyifeng.com/" target="_blank" rel="noopener noreferrer">http://es6.ruanyifeng.com/<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，教程版权归原作者所有。</p></blockquote> <h1 id="ecmascript-6-简介"><a href="#ecmascript-6-简介" class="header-anchor">#</a> ECMAScript 6 简介</h1> <p>ECMAScript 6.0（以下简称 ES6）是 JavaScript 语言的下一代标准，已经在 2015 年 6 月正式发布了。它的目标，是使得 JavaScript 语言可以用来编写复杂的大型应用程序，成为企业级开发语言。
    26. 

  26. </p> <h2 id="ecmascript-和-javascript-的关系"><a href="#ecmascript-和-javascript-的关系" class="header-anchor">#</a> ECMAScript 和 JavaScript 的关系</h2> <p>一个常见的问题是，ECMAScript 和 JavaScript 到底是什么关系？</p> <p>要讲清楚这个问题，需要回顾历史。1996 年 11 月，JavaScript 的创造者 Netscape 公司，决定将 JavaScript 提交给标准化组织 ECMA，希望这种语言能够成为国际标准。次年，ECMA 发布 262 号标准文件（ECMA-262）的第一版，规定了浏览器脚本语言的标准，并将这种语言称为 ECMAScript，这个版本就是 1.0 版。</p> <p>该标准从一开始就是针对 JavaScript 语言制定的，但是之所以不叫 JavaScript，有两个原因。一是商标，Java 是 Sun 公司的商标，根据授权协议，只有 Netscape 公司可以合法地使用 JavaScript 这个名字，且 JavaScript 本身也已经被 Netscape 公司注册为商标。二是想体现这门语言的制定者是 ECMA，不是 Netscape，这样有利于保证这门语言的开放性和中立性。</p> <p>因此，ECMAScript 和 JavaScript 的关系是，前者是后者的规格，后者是前者的一种实现（另外的 ECMAScript 方言还有 JScript 和 ActionScript）。日常场合，这两个词是可以互换的。</p> <h2 id="es6-与-ecmascript-2015-的关系"><a href="#es6-与-ecmascript-2015-的关系" class="header-anchor">#</a> ES6 与 ECMAScript 2015 的关系</h2> <p>ECMAScript 2015（简称 ES2015）这个词，也是经常可以看到的。它与 ES6 是什么关系呢？</p> <p>2011 年，ECMAScript 5.1 版发布后，就开始制定 6.0 版了。因此，ES6 这个词的原意，就是指 JavaScript 语言的下一个版本。</p> <p>但是，因为这个版本引入的语法功能太多，而且制定过程当中，还有很多组织和个人不断提交新功能。事情很快就变得清楚了，不可能在一个版本里面包括所有将要引入的功能。常规的做法是先发布 6.0 版，过一段时间再发 6.1 版，然后是 6.2 版、6.3 版等等。</p> <p>但是，标准的制定者不想这样做。他们想让标准的升级成为常规流程：任何人在任何时候，都可以向标准委员会提交新语法的提案，然后标准委员会每个月开一次会，评估这些提案是否可以接受，需要哪些改进。如果经过多次会议以后，一个提案足够成熟了，就可以正式进入标准了。这就是说，标准的版本升级成为了一个不断滚动的流程，每个月都会有变动。</p> <p>标准委员会最终决定，标准在每年的 6 月份正式发布一次，作为当年的正式版本。接下来的时间，就在这个版本的基础上做改动，直到下一年的 6 月份，草案就自然变成了新一年的版本。这样一来，就不需要以前的版本号了，只要用年份标记就可以了。</p> <p>ES6 的第一个版本，就这样在 2015 年 6 月发布了，正式名称就是《ECMAScript 2015 标准》（简称 ES2015）。2016 年 6 月，小幅修订的《ECMAScript 2016 标准》（简称 ES2016）如期发布，这个版本可以看作是 ES6.1 版，因为两者的差异非常小（只新增了数组实例的<code>includes</code>方法和指数运算符），基本上是同一个标准。根据计划，2017 年 6 月发布 ES2017 标准。</p> <p>因此，ES6 既是一个历史名词，也是一个泛指，含义是 5.1 版以后的 JavaScript 的下一代标准，涵盖了 ES2015、ES2016、ES2017 等等，而 ES2015 则是正式名称，特指该年发布的正式版本的语言标准。本书中提到 ES6 的地方，一般是指 ES2015 标准，但有时也是泛指“下一代 JavaScript 语言”。</p> <h2 id="语法提案的批准流程"><a href="#语法提案的批准流程" class="header-anchor">#</a> 语法提案的批准流程</h2> <p>任何人都可以向标准委员会（又称 TC39 委员会）提案，要求修改语言标准。</p> <p>一种新的语法从提案到变成正式标准，需要经历五个阶段。每个阶段的变动都需要由 TC39 委员会批准。</p> <ul><li>Stage 0 - Strawman（展示阶段）</li> <li>Stage 1 - Proposal（征求意见阶段）</li> <li>Stage 2 - Draft（草案阶段）</li> <li>Stage 3 - Candidate（候选人阶段）</li> <li>Stage 4 - Finished（定案阶段）</li></ul> <p>一个提案只要能进入 Stage 2，就差不多肯定会包括在以后的正式标准里面。ECMAScript 当前的所有提案，可以在 TC39 的官方网站<a href="https://github.com/tc39/ecma262" target="_blank" rel="noopener noreferrer">GitHub.com/tc39/ecma262<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>查看。</p> <p>本书的写作目标之一，是跟踪 ECMAScript 语言的最新进展，介绍 5.1 版本以后所有的新语法。对于那些明确或很有希望，将要列入标准的新语法，都将予以介绍。</p> <h2 id="ecmascript-的历史"><a href="#ecmascript-的历史" class="header-anchor">#</a> ECMAScript 的历史</h2> <p>ES6 从开始制定到最后发布，整整用了 15 年。</p> <p>前面提到，ECMAScript 1.0 是 1997 年发布的，接下来的两年，连续发布了 ECMAScript 2.0（1998 年 6 月）和 ECMAScript 3.0（1999 年 12 月）。3.0 版是一个巨大的成功，在业界得到广泛支持，成为通行标准，奠定了 JavaScript 语言的基本语法，以后的版本完全继承。直到今天，初学者一开始学习 JavaScript，其实就是在学 3.0 版的语法。</p> <p>2000 年，ECMAScript 4.0 开始酝酿。这个版本最后没有通过，但是它的大部分内容被 ES6 继承了。因此，ES6 制定的起点其实是 2000 年。</p> <p>为什么 ES4 没有通过呢？因为这个版本太激进了，对 ES3 做了彻底升级，导致标准委员会的一些成员不愿意接受。ECMA 的第 39 号技术专家委员会（Technical Committee 39，简称 TC39）负责制订 ECMAScript 标准，成员包括 Microsoft、Mozilla、Google 等大公司。</p> <p>2007 年 10 月，ECMAScript 4.0 版草案发布，本来预计次年 8 月发布正式版本。但是，各方对于是否通过这个标准，发生了严重分歧。以 Yahoo、Microsoft、Google 为首的大公司，反对 JavaScript 的大幅升级，主张小幅改动；以 JavaScript 创造者 Brendan Eich 为首的 Mozilla 公司，则坚持当前的草案。</p> <p>2008 年 7 月，由于对于下一个版本应该包括哪些功能，各方分歧太大，争论过于激烈，ECMA 开会决定，中止 ECMAScript 4.0 的开发，将其中涉及现有功能改善的一小部分，发布为 ECMAScript 3.1，而将其他激进的设想扩大范围，放入以后的版本，由于会议的气氛，该版本的项目代号起名为 Harmony（和谐）。会后不久，ECMAScript 3.1 就改名为 ECMAScript 5。</p> <p>2009 年 12 月，ECMAScript 5.0 版正式发布。Harmony 项目则一分为二，一些较为可行的设想定名为 JavaScript.next 继续开发，后来演变成 ECMAScript 6；一些不是很成熟的设想，则被视为 JavaScript.next.next，在更远的将来再考虑推出。TC39 委员会的总体考虑是，ES5 与 ES3 基本保持兼容，较大的语法修正和新功能加入，将由 JavaScript.next 完成。当时，JavaScript.next 指的是 ES6，第六版发布以后，就指 ES7。TC39 的判断是，ES5 会在 2013 年的年中成为 JavaScript 开发的主流标准，并在此后五年中一直保持这个位置。</p> <p>2011 年 6 月，ECMAScript 5.1 版发布，并且成为 ISO 国际标准（ISO/IEC 16262:2011）。</p> <p>2013 年 3 月，ECMAScript 6 草案冻结，不再添加新功能。新的功能设想将被放到 ECMAScript 7。</p> <p>2013 年 12 月，ECMAScript 6 草案发布。然后是 12 个月的讨论期，听取各方反馈。</p> <p>2015 年 6 月，ECMAScript 6 正式通过，成为国际标准。从 2000 年算起，这时已经过去了 15 年。</p> <h2 id="部署进度"><a href="#部署进度" class="header-anchor">#</a> 部署进度</h2> <p>各大浏览器的最新版本，对 ES6 的支持可以查看<a href="https://kangax.github.io/compat-table/es6/" target="_blank" rel="noopener noreferrer">kangax.github.io/compat-table/es6/<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。随着时间的推移，支持度已经越来越高了，超过 90%的 ES6 语法特性都实现了。</p> <p>Node 是 JavaScript 的服务器运行环境（runtime）。它对 ES6 的支持度更高。除了那些默认打开的功能，还有一些语法功能已经实现了，但是默认没有打开。使用下面的命令，可以查看 Node 已经实现的 ES6 特性。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>// Linux <span class="token operator">&amp;</span> Mac
    27. 

  27. $ <span class="token function">node</span> --v8-options <span class="token operator">|</span> <span class="token function">grep</span> harmony
    28. 

  28. 

  29. // Windows
    30. 

  30. $ <span class="token function">node</span> --v8-options <span class="token operator">|</span> findstr harmony
    31. 

  31. </code></pre> <div class="line-numbers-wrapper"><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></div></div><p>我写了一个工具 <a href="https://github.com/ruanyf/es-checker" target="_blank" rel="noopener noreferrer">ES-Checker<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，用来检查各种运行环境对 ES6 的支持情况。访问<a href="http://ruanyf.github.io/es-checker" target="_blank" rel="noopener noreferrer">ruanyf.github.io/es-checker<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，可以看到您的浏览器支持 ES6 的程度。运行下面的命令，可以查看你正在使用的 Node 环境对 ES6 的支持程度。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> <span class="token function">install</span> <span class="token parameter variable">-g</span> es-checker
    32. 

  32. $ es-checker
    33. 

  33. 

  34. <span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">=</span>
    35. 

  35. Passes <span class="token number">24</span> feature Detections
    36. 

  36. Your runtime supports <span class="token number">57</span>% of ECMAScript <span class="token number">6</span>
    37. 

  37. <span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">==</span><span class="token operator">=</span>
    38. 

  38. </code></pre> <div class="line-numbers-wrapper"><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><h2 id="babel-转码器"><a href="#babel-转码器" class="header-anchor">#</a> Babel 转码器</h2> <p><a href="https://babeljs.io/" target="_blank" rel="noopener noreferrer">Babel<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 是一个广泛使用的 ES6 转码器，可以将 ES6 代码转为 ES5 代码，从而在现有环境执行。这意味着，你可以用 ES6 的方式编写程序，又不用担心现有环境是否支持。下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 转码前</span>
    39. 

  39. input<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">item</span> <span class="token operator">=&gt;</span> item <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    40. 

  40. 

  41. <span class="token comment">// 转码后</span>
    42. 

  42. input<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    43. 

  43.   <span class="token keyword">return</span> item <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span>
    44. 

  44. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    45. 

  45. </code></pre> <div class="line-numbers-wrapper"><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>上面的原始代码用了箭头函数，Babel 将其转为普通函数，就能在不支持箭头函数的 JavaScript 环境执行了。</p> <p>下面的命令在项目目录中，安装 Babel。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> <span class="token function">install</span> --save-dev @babel/core
    46. 

  46. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="配置文件-babelrc"><a href="#配置文件-babelrc" class="header-anchor">#</a> 配置文件<code>.babelrc</code></h3> <p>Babel 的配置文件是<code>.babelrc</code>，存放在项目的根目录下。使用 Babel 的第一步，就是配置这个文件。</p> <p>该文件用来设置转码规则和插件，基本格式如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
    47. 

  47.   <span class="token string-property property">&quot;presets&quot;</span><span class="token operator">:</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
    48. 

  48.   <span class="token string-property property">&quot;plugins&quot;</span><span class="token operator">:</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>
    49. 

  49. <span class="token punctuation">}</span>
    50. 

  50. </code></pre> <div class="line-numbers-wrapper"><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></div></div><p><code>presets</code>字段设定转码规则，官方提供以下的规则集，你可以根据需要安装。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code><span class="token comment"># 最新转码规则</span>
    51. 

  51. $ <span class="token function">npm</span> <span class="token function">install</span> --save-dev @babel/preset-env
    52. 

  52. 

  53. <span class="token comment"># react 转码规则</span>
    54. 

  54. $ <span class="token function">npm</span> <span class="token function">install</span> --save-dev @babel/preset-react
    55. 

  55. </code></pre> <div class="line-numbers-wrapper"><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></div></div><p>然后，将这些规则加入<code>.babelrc</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>  <span class="token punctuation">{</span>
    56. 

  56.     <span class="token string-property property">&quot;presets&quot;</span><span class="token operator">:</span> <span class="token punctuation">[</span>
    57. 

  57.       <span class="token string">&quot;@babel/env&quot;</span><span class="token punctuation">,</span>
    58. 

  58.       <span class="token string">&quot;@babel/preset-react&quot;</span>
    59. 

  59.     <span class="token punctuation">]</span><span class="token punctuation">,</span>
    60. 

  60.     <span class="token string-property property">&quot;plugins&quot;</span><span class="token operator">:</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>
    61. 

  61.   <span class="token punctuation">}</span>
    62. 

  62. </code></pre> <div class="line-numbers-wrapper"><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>注意，以下所有 Babel 工具和模块的使用，都必须先写好<code>.babelrc</code>。</p> <h3 id="命令行转码"><a href="#命令行转码" class="header-anchor">#</a> 命令行转码</h3> <p>Babel 提供命令行工具<code>@babel/cli</code>，用于命令行转码。</p> <p>它的安装命令如下。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> <span class="token function">install</span> --save-dev @babel/cli
    63. 

  63. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>基本用法如下。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code><span class="token comment"># 转码结果输出到标准输出</span>
    64. 

  64. $ npx babel example.js
    65. 

  65. 

  66. <span class="token comment"># 转码结果写入一个文件</span>
    67. 

  67. <span class="token comment"># --out-file 或 -o 参数指定输出文件</span>
    68. 

  68. $ npx babel example.js --out-file compiled.js
    69. 

  69. <span class="token comment"># 或者</span>
    70. 

  70. $ npx babel example.js <span class="token parameter variable">-o</span> compiled.js
    71. 

  71. 

  72. <span class="token comment"># 整个目录转码</span>
    73. 

  73. <span class="token comment"># --out-dir 或 -d 参数指定输出目录</span>
    74. 

  74. $ npx babel src --out-dir lib
    75. 

  75. <span class="token comment"># 或者</span>
    76. 

  76. $ npx babel src <span class="token parameter variable">-d</span> lib
    77. 

  77. 

  78. <span class="token comment"># -s 参数生成source map文件</span>
    79. 

  79. $ npx babel src <span class="token parameter variable">-d</span> lib <span class="token parameter variable">-s</span>
    80. 

  80. </code></pre> <div class="line-numbers-wrapper"><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><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><h3 id="babel-node"><a href="#babel-node" class="header-anchor">#</a> babel-node</h3> <p><code>@babel/node</code>模块的<code>babel-node</code>命令，提供一个支持 ES6 的 REPL 环境。它支持 Node 的 REPL 环境的所有功能，而且可以直接运行 ES6 代码。</p> <p>首先，安装这个模块。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> <span class="token function">install</span> --save-dev @babel/node
    81. 

  81. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>然后，执行<code>babel-node</code>就进入 REPL 环境。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ npx babel-node
    82. 

  82. <span class="token operator">&gt;</span> <span class="token punctuation">(</span>x <span class="token operator">=</span><span class="token operator">&gt;</span> x * <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
    83. 

  83. <span class="token number">2</span>
    84. 

  84. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>babel-node</code>命令可以直接运行 ES6 脚本。将上面的代码放入脚本文件<code>es6.js</code>，然后直接运行。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code><span class="token comment"># es6.js 的代码</span>
    85. 

  85. <span class="token comment"># console.log((x =&gt; x * 2)(1));</span>
    86. 

  86. $ npx babel-node es6.js
    87. 

  87. <span class="token number">2</span>
    88. 

  88. </code></pre> <div class="line-numbers-wrapper"><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></div></div><h3 id="babel-register-模块"><a href="#babel-register-模块" class="header-anchor">#</a> @babel/register 模块</h3> <p><code>@babel/register</code>模块改写<code>require</code>命令，为它加上一个钩子。此后，每当使用<code>require</code>加载<code>.js</code>、<code>.jsx</code>、<code>.es</code>和<code>.es6</code>后缀名的文件，就会先用 Babel 进行转码。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> <span class="token function">install</span> --save-dev @babel/register
    89. 

  89. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>使用时，必须首先加载<code>@babel/register</code>。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>// index.js
    90. 

  90. require<span class="token punctuation">(</span><span class="token string">'@babel/register'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    91. 

  91. require<span class="token punctuation">(</span><span class="token string">'./es6.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    92. 

  92. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>然后，就不需要手动对<code>index.js</code>转码了。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">node</span> index.js
    93. 

  93. <span class="token number">2</span>
    94. 

  94. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>需要注意的是，<code>@babel/register</code>只会对<code>require</code>命令加载的文件转码，而不会对当前文件转码。另外，由于它是实时转码，所以只适合在开发环境使用。</p> <h3 id="babel-api"><a href="#babel-api" class="header-anchor">#</a> babel API</h3> <p>如果某些代码需要调用 Babel 的 API 进行转码，就要使用<code>@babel/core</code>模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> babel <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'@babel/core'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    95. 

  95. 

  96. <span class="token comment">// 字符串转码</span>
    97. 

  97. babel<span class="token punctuation">.</span><span class="token function">transform</span><span class="token punctuation">(</span><span class="token string">'code();'</span><span class="token punctuation">,</span> options<span class="token punctuation">)</span><span class="token punctuation">;</span>
    98. 

  98. <span class="token comment">// =&gt; { code, map, ast }</span>
    99. 

  99. 

  100. <span class="token comment">// 文件转码（异步）</span>
    101. 

  101. babel<span class="token punctuation">.</span><span class="token function">transformFile</span><span class="token punctuation">(</span><span class="token string">'filename.js'</span><span class="token punctuation">,</span> options<span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">err<span class="token punctuation">,</span> result</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    102. 

  102.   result<span class="token punctuation">;</span> <span class="token comment">// =&gt; { code, map, ast }</span>
    103. 

  103. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    104. 

  104. 

  105. <span class="token comment">// 文件转码（同步）</span>
    106. 

  106. babel<span class="token punctuation">.</span><span class="token function">transformFileSync</span><span class="token punctuation">(</span><span class="token string">'filename.js'</span><span class="token punctuation">,</span> options<span class="token punctuation">)</span><span class="token punctuation">;</span>
    107. 

  107. <span class="token comment">// =&gt; { code, map, ast }</span>
    108. 

  108. 

  109. <span class="token comment">// Babel AST转码</span>
    110. 

  110. babel<span class="token punctuation">.</span><span class="token function">transformFromAst</span><span class="token punctuation">(</span>ast<span class="token punctuation">,</span> code<span class="token punctuation">,</span> options<span class="token punctuation">)</span><span class="token punctuation">;</span>
    111. 

  111. <span class="token comment">// =&gt; { code, map, ast }</span>
    112. 

  112. </code></pre> <div class="line-numbers-wrapper"><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><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>配置对象<code>options</code>，可以参看官方文档<a href="http://babeljs.io/docs/usage/options/" target="_blank" rel="noopener noreferrer">http://babeljs.io/docs/usage/options/<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。</p> <p>下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> es6Code <span class="token operator">=</span> <span class="token string">'let x = n =&gt; n + 1'</span><span class="token punctuation">;</span>
    113. 

  113. <span class="token keyword">var</span> es5Code <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'@babel/core'</span><span class="token punctuation">)</span>
    114. 

  114.   <span class="token punctuation">.</span><span class="token function">transform</span><span class="token punctuation">(</span>es6Code<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    115. 

  115.     <span class="token literal-property property">presets</span><span class="token operator">:</span> <span class="token punctuation">[</span><span class="token string">'@babel/env'</span><span class="token punctuation">]</span>
    116. 

  116.   <span class="token punctuation">}</span><span class="token punctuation">)</span>
    117. 

  117.   <span class="token punctuation">.</span>code<span class="token punctuation">;</span>
    118. 

  118. 

  119. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>es5Code<span class="token punctuation">)</span><span class="token punctuation">;</span>
    120. 

  120. <span class="token comment">// '&quot;use strict&quot;;\n\nvar x = function x(n) {\n  return n + 1;\n};'</span>
    121. 

  121. </code></pre> <div class="line-numbers-wrapper"><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></div></div><p>上面代码中，<code>transform</code>方法的第一个参数是一个字符串，表示需要被转换的 ES6 代码，第二个参数是转换的配置对象。</p> <h3 id="babel-polyfill"><a href="#babel-polyfill" class="header-anchor">#</a> @babel/polyfill</h3> <p>Babel 默认只转换新的 JavaScript 句法（syntax），而不转换新的 API，比如<code>Iterator</code>、<code>Generator</code>、<code>Set</code>、<code>Map</code>、<code>Proxy</code>、<code>Reflect</code>、<code>Symbol</code>、<code>Promise</code>等全局对象，以及一些定义在全局对象上的方法（比如<code>Object.assign</code>）都不会转码。</p> <p>举例来说，ES6 在<code>Array</code>对象上新增了<code>Array.from</code>方法。Babel 就不会转码这个方法。如果想让这个方法运行，必须使用<code>babel-polyfill</code>，为当前环境提供一个垫片。</p> <p>安装命令如下。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> <span class="token function">install</span> --save-dev @babel/polyfill
    122. 

  122. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>然后，在脚本头部，加入如下一行代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token string">'@babel/polyfill'</span><span class="token punctuation">;</span>
    123. 

  123. <span class="token comment">// 或者</span>
    124. 

  124. <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'@babel/polyfill'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    125. 

  125. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>Babel 默认不转码的 API 非常多，详细清单可以查看<code>babel-plugin-transform-runtime</code>模块的<a href="https://github.com/babel/babel/blob/master/packages/babel-plugin-transform-runtime/src/definitions.js" target="_blank" rel="noopener noreferrer">definitions.js<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>文件。</p> <h3 id="浏览器环境"><a href="#浏览器环境" class="header-anchor">#</a> 浏览器环境</h3> <p>Babel 也可以用于浏览器环境，使用<a href="https://babeljs.io/docs/en/next/babel-standalone.html" target="_blank" rel="noopener noreferrer">@babel/standalone<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>模块提供的浏览器版本，将其插入网页。</p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>https://unpkg.com/@babel/standalone/babel.min.js<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    126. 

  126. <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>text/babel<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    127. 

  127. <span class="token comment">// Your ES6 code</span>
    128. 

  128. </span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    129. 

  129. </code></pre> <div class="line-numbers-wrapper"><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></div></div><p>注意，网页实时将 ES6 代码转为 ES5，对性能会有影响。生产环境需要加载已经转码完成的脚本。</p> <p>Babel 提供一个<a href="https://babeljs.io/repl/" target="_blank" rel="noopener noreferrer">REPL 在线编译器<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，可以在线将 ES6 代码转为 ES5 代码。转换后的代码，可以直接作为 ES5 代码插入网页运行。</p> <h2 id="traceur-转码器"><a href="#traceur-转码器" class="header-anchor">#</a> Traceur 转码器</h2> <p>Google 公司的<a href="https://github.com/google/traceur-compiler" target="_blank" rel="noopener noreferrer">Traceur<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>转码器，也可以将 ES6 代码转为 ES5 代码。</p> <h3 id="直接插入网页"><a href="#直接插入网页" class="header-anchor">#</a> 直接插入网页</h3> <p>Traceur 允许将 ES6 代码直接插入网页。首先，必须在网页头部加载 Traceur 库文件。</p> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>https://google.github.io/traceur-compiler/bin/traceur.js<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    130. 

  130. <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>https://google.github.io/traceur-compiler/bin/BrowserSystem.js<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    131. 

  131. <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">src</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>https://google.github.io/traceur-compiler/src/bootstrap.js<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    132. 

  132. <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>module<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    133. 

  133.   <span class="token keyword">import</span> <span class="token string">'./Greeter.js'</span><span class="token punctuation">;</span>
    134. 

  134. </span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
    135. 

  135. </code></pre> <div class="line-numbers-wrapper"><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>上面代码中，一共有 4 个<code>script</code>标签。第一个是加载 Traceur 的库文件，第二个和第三个是将这个库文件用于浏览器环境，第四个则是加载用户脚本，这个脚本里面可以使用 ES6 代码。</p> <p>注意，第四个<code>script</code>标签的<code>type</code>属性的值是<code>module</code>，而不是<code>text/javascript</code>。这是 Traceur 编译器识别 ES6 代码的标志，编译器会自动将所有<code>type=module</code>的代码编译为 ES5，然后再交给浏览器执行。</p> <p>除了引用外部 ES6 脚本，也可以直接在网页中放置 ES6 代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token operator">&lt;</span>script type<span class="token operator">=</span><span class="token string">&quot;module&quot;</span><span class="token operator">&gt;</span>
    136. 

  136.   <span class="token keyword">class</span> <span class="token class-name">Calc</span> <span class="token punctuation">{</span>
    137. 

  137.     <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    138. 

  138.       console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Calc constructor'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    139. 

  139.     <span class="token punctuation">}</span>
    140. 

  140.     <span class="token function">add</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    141. 

  141.       <span class="token keyword">return</span> a <span class="token operator">+</span> b<span class="token punctuation">;</span>
    142. 

  142.     <span class="token punctuation">}</span>
    143. 

  143.   <span class="token punctuation">}</span>
    144. 

  144. 

  145.   <span class="token keyword">var</span> c <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Calc</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    146. 

  146.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>c<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    147. 

  147. <span class="token operator">&lt;</span><span class="token operator">/</span>script<span class="token operator">&gt;</span>
    148. 

  148. </code></pre> <div class="line-numbers-wrapper"><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><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>正常情况下，上面代码会在控制台打印出<code>9</code>。</p> <p>如果想对 Traceur 的行为有精确控制，可以采用下面参数配置的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token operator">&lt;</span>script<span class="token operator">&gt;</span>
    149. 

  149.   <span class="token comment">// Create the System object</span>
    150. 

  150.   window<span class="token punctuation">.</span>System <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">traceur<span class="token punctuation">.</span>runtime<span class="token punctuation">.</span>BrowserTraceurLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    151. 

  151.   <span class="token comment">// Set some experimental options</span>
    152. 

  152.   <span class="token keyword">var</span> metadata <span class="token operator">=</span> <span class="token punctuation">{</span>
    153. 

  153.     <span class="token literal-property property">traceurOptions</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    154. 

  154.       <span class="token literal-property property">experimental</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    155. 

  155.       <span class="token literal-property property">properTailCalls</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    156. 

  156.       <span class="token literal-property property">symbols</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    157. 

  157.       <span class="token literal-property property">arrayComprehension</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    158. 

  158.       <span class="token literal-property property">asyncFunctions</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    159. 

  159.       <span class="token literal-property property">asyncGenerators</span><span class="token operator">:</span> exponentiation<span class="token punctuation">,</span>
    160. 

  160.       <span class="token literal-property property">forOn</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    161. 

  161.       <span class="token literal-property property">generatorComprehension</span><span class="token operator">:</span> <span class="token boolean">true</span>
    162. 

  162.     <span class="token punctuation">}</span>
    163. 

  163.   <span class="token punctuation">}</span><span class="token punctuation">;</span>
    164. 

  164.   <span class="token comment">// Load your module</span>
    165. 

  165.   System<span class="token punctuation">.</span><span class="token function">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">metadata</span><span class="token operator">:</span> metadata<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">ex</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    166. 

  166.     console<span class="token punctuation">.</span><span class="token function">error</span><span class="token punctuation">(</span><span class="token string">'Import failed'</span><span class="token punctuation">,</span> ex<span class="token punctuation">.</span>stack <span class="token operator">||</span> ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
    167. 

  167.   <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    168. 

  168. <span class="token operator">&lt;</span><span class="token operator">/</span>script<span class="token operator">&gt;</span>
    169. 

  169. </code></pre> <div class="line-numbers-wrapper"><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><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p>上面代码中，首先生成 Traceur 的全局对象<code>window.System</code>，然后<code>System.import</code>方法可以用来加载 ES6。加载的时候，需要传入一个配置对象<code>metadata</code>，该对象的<code>traceurOptions</code>属性可以配置支持 ES6 功能。如果设为<code>experimental: true</code>，就表示除了 ES6 以外，还支持一些实验性的新功能。</p> <h3 id="在线转换"><a href="#在线转换" class="header-anchor">#</a> 在线转换</h3> <p>Traceur 也提供一个<a href="http://google.github.io/traceur-compiler/demo/repl.html" target="_blank" rel="noopener noreferrer">在线编译器<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，可以在线将 ES6 代码转为 ES5 代码。转换后的代码，可以直接作为 ES5 代码插入网页运行。</p> <p>上面的例子转为 ES5 代码运行，就是下面这个样子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token operator">&lt;</span>script src<span class="token operator">=</span><span class="token string">&quot;https://google.github.io/traceur-compiler/bin/traceur.js&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>script<span class="token operator">&gt;</span>
    170. 

  170. <span class="token operator">&lt;</span>script src<span class="token operator">=</span><span class="token string">&quot;https://google.github.io/traceur-compiler/bin/BrowserSystem.js&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>script<span class="token operator">&gt;</span>
    171. 

  171. <span class="token operator">&lt;</span>script src<span class="token operator">=</span><span class="token string">&quot;https://google.github.io/traceur-compiler/src/bootstrap.js&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>script<span class="token operator">&gt;</span>
    172. 

  172. <span class="token operator">&lt;</span>script<span class="token operator">&gt;</span>
    173. 

  173. $traceurRuntime<span class="token punctuation">.</span>ModuleStore<span class="token punctuation">.</span><span class="token function">getAnonymousModule</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    174. 

  174.   <span class="token string">&quot;use strict&quot;</span><span class="token punctuation">;</span>
    175. 

  175. 

  176.   <span class="token keyword">var</span> <span class="token function-variable function">Calc</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token function">Calc</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    177. 

  177.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Calc constructor'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    178. 

  178.   <span class="token punctuation">}</span><span class="token punctuation">;</span>
    179. 

  179. 

  180.   <span class="token punctuation">(</span>$traceurRuntime<span class="token punctuation">.</span>createClass<span class="token punctuation">)</span><span class="token punctuation">(</span>Calc<span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token function-variable function">add</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    181. 

  181.     <span class="token keyword">return</span> a <span class="token operator">+</span> b<span class="token punctuation">;</span>
    182. 

  182.   <span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    183. 

  183. 

  184.   <span class="token keyword">var</span> c <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Calc</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    185. 

  185.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>c<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    186. 

  186.   <span class="token keyword">return</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    187. 

  187. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    188. 

  188. <span class="token operator">&lt;</span><span class="token operator">/</span>script<span class="token operator">&gt;</span>
    189. 

  189. </code></pre> <div class="line-numbers-wrapper"><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><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><h3 id="命令行转换"><a href="#命令行转换" class="header-anchor">#</a> 命令行转换</h3> <p>作为命令行工具使用时，Traceur 是一个 Node 的模块，首先需要用 npm 安装。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> <span class="token function">install</span> <span class="token parameter variable">-g</span> traceur
    190. 

  190. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>安装成功后，就可以在命令行下使用 Traceur 了。</p> <p>Traceur 直接运行 ES6 脚本文件，会在标准输出显示运行结果，以前面的<code>calc.js</code>为例。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ traceur calc.js
    191. 

  191. Calc constructor
    192. 

  192. <span class="token number">9</span>
    193. 

  193. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>如果要将 ES6 脚本转为 ES5 保存，要采用下面的写法。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ traceur <span class="token parameter variable">--script</span> calc.es6.js <span class="token parameter variable">--out</span> calc.es5.js
    194. 

  194. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码的<code>--script</code>选项表示指定输入文件，<code>--out</code>选项表示指定输出文件。</p> <p>为了防止有些特性编译不成功，最好加上<code>--experimental</code>选项。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ traceur <span class="token parameter variable">--script</span> calc.es6.js <span class="token parameter variable">--out</span> calc.es5.js <span class="token parameter variable">--experimental</span>
    195. 

  195. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>命令行下转换生成的文件，就可以直接放到浏览器中运行。</p> <h3 id="node-环境的用法"><a href="#node-环境的用法" class="header-anchor">#</a> Node 环境的用法</h3> <p>Traceur 的 Node 用法如下（假定已安装<code>traceur</code>模块）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> traceur <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'traceur'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    196. 

  196. <span class="token keyword">var</span> fs <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'fs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    197. 

  197. 

  198. <span class="token comment">// 将 ES6 脚本转为字符串</span>
    199. 

  199. <span class="token keyword">var</span> contents <span class="token operator">=</span> fs<span class="token punctuation">.</span><span class="token function">readFileSync</span><span class="token punctuation">(</span><span class="token string">'es6-file.js'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    200. 

  200. 

  201. <span class="token keyword">var</span> result <span class="token operator">=</span> traceur<span class="token punctuation">.</span><span class="token function">compile</span><span class="token punctuation">(</span>contents<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    202. 

  202.   <span class="token literal-property property">filename</span><span class="token operator">:</span> <span class="token string">'es6-file.js'</span><span class="token punctuation">,</span>
    203. 

  203.   <span class="token literal-property property">sourceMap</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    204. 

  204.   <span class="token comment">// 其他设置</span>
    205. 

  205.   <span class="token literal-property property">modules</span><span class="token operator">:</span> <span class="token string">'commonjs'</span>
    206. 

  206. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    207. 

  207. 

  208. <span class="token keyword">if</span> <span class="token punctuation">(</span>result<span class="token punctuation">.</span>error<span class="token punctuation">)</span>
    209. 

  209.   <span class="token keyword">throw</span> result<span class="token punctuation">.</span>error<span class="token punctuation">;</span>
    210. 

  210. 

  211. <span class="token comment">// result 对象的 js 属性就是转换后的 ES5 代码</span>
    212. 

  212. fs<span class="token punctuation">.</span><span class="token function">writeFileSync</span><span class="token punctuation">(</span><span class="token string">'out.js'</span><span class="token punctuation">,</span> result<span class="token punctuation">.</span>js<span class="token punctuation">)</span><span class="token punctuation">;</span>
    213. 

  213. <span class="token comment">// sourceMap 属性对应 map 文件</span>
    214. 

  214. fs<span class="token punctuation">.</span><span class="token function">writeFileSync</span><span class="token punctuation">(</span><span class="token string">'out.js.map'</span><span class="token punctuation">,</span> result<span class="token punctuation">.</span>sourceMap<span class="token punctuation">)</span><span class="token punctuation">;</span>
    215. 

  215. </code></pre> <div class="line-numbers-wrapper"><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><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《ES6 教程》笔记/01.ECMAScript 6 简介.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=ES6" title="标签">#ES6</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><!----> <a href="/blog/pages/c1edd70a6b7c7872/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">let 和 const 命令</div></a></div> <div class="page-nav"><p class="inner"><!----> <span class="next"><a href="/blog/pages/c1edd70a6b7c7872/">let 和 const 命令</a>→
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