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ECMAScript 2017 新特性一览
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<p>本文主要参考自 <code>2ality</code> 的文章：<a href="http://2ality.com/2016/02/ecmascript-2017.html" target="_blank" rel="noopener">ECMAScript 2017 (ES8): the final feature set</a>。部分章节会有修改，加入自己的一些理解与观点。<em>阅读本文需要有一定的 ES 基础</em>。</p>
</blockquote>
<blockquote>
<p>官方公告：<a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm" target="_blank" rel="noopener">Standard ECMA-262</a>。</p>
</blockquote>
<h2 id="主要特性"><a href="#主要特性" class="headerlink" title="主要特性"></a>主要特性</h2><h3 id="异步函数"><a href="#异步函数" class="headerlink" title="异步函数"></a>异步函数</h3><p>ES6 发布的时候，引入了 <code>Promise</code> 来处理异步操作，一个常见的 Async 函数如下：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fetchJson</span>(<span class="params">url</span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> fetch(url)</span><br><span class="line"> .then(<span class="function"><span class="params">request</span> =></span> request.text())</span><br><span class="line"> .then(<span class="function"><span class="params">text</span> =></span> {</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">JSON</span>.parse(text);</span><br><span class="line"> })</span><br><span class="line"> .catch(<span class="function"><span class="params">error</span> =></span> {</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">`ERROR: <span class="subst">${error.stack}</span>`</span>);</span><br><span class="line"> });</span><br><span class="line">}</span><br><span class="line">fetchJson(<span class="string">'http://example.com/some_file.json'</span>)</span><br><span class="line">.then(<span class="function"><span class="params">obj</span> =></span> <span class="built_in">console</span>.log(obj));</span><br></pre></td></tr></table></figure>
<p>现在，使用 ES2017 中新增的关键字 <code>async</code> 和 <code>await</code>，我们可以实现以看起来像是同步的代码来处理异步的操作。如上面的代码可以写成：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">fetchData</span>(<span class="params">url</span>) </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">let</span> request = <span class="keyword">await</span> fetch(url);</span><br><span class="line"> <span class="keyword">let</span> text = <span class="keyword">await</span> request.text();</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">JSON</span>.parse(text);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">catch</span> (err) {</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">`Error: <span class="subst">${err.stack}</span>`</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>异步函数还有一些变体，罗列如下：</p>
<ul>
<li>异步函数声明：<code>async function foo() {}</code></li>
<li>异步函数表达式：<code>const foo = async function () {};</code></li>
<li>异步方法定义：<code>let obj = {async foo () {}};</code></li>
<li>异步箭头函数：<code>const foo = async () => {};</code></li>
</ul>
<p>更多资料可参阅：<a href="https://github.com/tc39/ecmascript-asyncawait" target="_blank" rel="noopener">tc39/ecmascript-asyncawait: Async/await for ECMAScript</a>，<a href="http://exploringjs.com/es6/ch_generators.html#sec_co-library" target="_blank" rel="noopener">Simplifying asynchronous computations via generators (section in “Exploring ES6”)</a></p>
<h3 id="共享内存和原子"><a href="#共享内存和原子" class="headerlink" title="共享内存和原子"></a>共享内存和原子</h3><p>共享内存 <code>shared memory</code> 主要是处理并行事件对于资源调用的一种机制。</p>
<p>该功能引入一个新的低级别 <code>Atomics</code> 命名空间对象和一个 <code>Shared Array Buffer</code> 构造函数来作为高级别并发抽象的原始构建块。这样可以使得开发人员可以使用多个 <code>service worker</code> 和核心线程之间的 <code>Shared Array Buffer</code> 对象的数据。通过这种方式，可以更轻松的在 <code>worker</code> 之间进行数据共享，改善它们之间的协调性。</p>
<ul>
<li><a href="https://github.com/tc39/ecmascript_sharedmem" target="_blank" rel="noopener">tc39/ecmascript_sharedmem: Shared memory and atomics for ECMAscript</a></li>
<li><a href="http://2ality.com/2017/01/shared-array-buffer.html" target="_blank" rel="noopener">ES proposal: Shared memory and atomics</a></li>
</ul>
<h2 id="次要更新"><a href="#次要更新" class="headerlink" title="次要更新"></a>次要更新</h2><h3 id="Object-values-Object-entries"><a href="#Object-values-Object-entries" class="headerlink" title="Object.values/Object.entries"></a>Object.values/Object.entries</h3><p>对象为键值对的数据结构时，每一个键值对都是一个 <code>entry</code>。<code>Object.entries</code> 提供将对象转换为其可枚举的每一个对象的集合的方法。而 <code>Object.values</code> 提取出对象中可枚举的字符串键值属性的所有值。</p>
<p><code>Object.values</code> 跟 <code>Object.keys</code> 功能类似。</p>
<p>举个例子，有一个这样的对象：<code>foo = {a: 1, b: 2, c: 3};</code>，上面提到的三个方法的结果分别为：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line">> foo = {a:1,b:2,c:3}</span><br><span class="line">< Object {a: 1, b: 2, c: 3}</span><br><span class="line">> Object.values(foo)</span><br><span class="line">< (3) [1, 2, 3]</span><br><span class="line">> Object.keys(foo)</span><br><span class="line">< (3) ["a", "b", "c"]</span><br><span class="line">> Object.entries(foo)</span><br><span class="line">< (3) [Array(2), Array(2), Array(2)]0: Array(2)1: Array(2)2: Array(2)length: 3__proto__: Array(0)</span><br><span class="line"></span><br><span class="line">------</span><br><span class="line">> Object.entries({ one: 1, two: 2 })</span><br><span class="line">[ [ 'one', 1 ], [ 'two', 2 ] ]</span><br><span class="line"></span><br><span class="line">> Object.entries({ [Symbol()]: 123, foo: 'abc' });</span><br><span class="line">[ [ 'foo', 'abc' ] ]</span><br></pre></td></tr></table></figure>
<ul>
<li><a href="http://2ality.com/2015/11/stage3-object-entries.html" target="_blank" rel="noopener">ES proposal: Object.entries() and Object.values()</a></li>
<li><a href="https://github.com/tc39/proposal-object-values-entries" target="_blank" rel="noopener">Object.values/Object.entries</a></li>
</ul>
<h3 id="字符串填充"><a href="#字符串填充" class="headerlink" title="字符串填充"></a>字符串填充</h3><p>针对 <code>String</code> 对象，引入了 <code>String Padding</code> 的规范，为字符串的处理添加了两个字符串填充的方法：<code>padStart</code> 和 <code>padEnd</code>。</p>
<h4 id="String-prototype-padStart"><a href="#String-prototype-padStart" class="headerlink" title="String.prototype.padStart"></a>String.prototype.padStart</h4><p>字符串的头部填充。接收两个参数 <code>String.prototype.padStart(maxLength, fillString=' ')</code>，第一个参数为填充的最大长度，第二个参数为指定填充的字符串。</p>
<p>假设我们有一个这样的操作：<code>s.padStart(n, f)</code>，s 为待填充的字符串，n 为填充后的长度，f 为填充的字符串。如果 f 的长度不够 n，则会重复使用，直到填充的长度达到了 n。如果 s 本身的长度已经超过 n 则不会进行填充。</p>
<h4 id="String-prototype-padEnd"><a href="#String-prototype-padEnd" class="headerlink" title="String.prototype.padEnd"></a>String.prototype.padEnd</h4><p>跟上面的方法类似，填充的位置在后面，其他处理方式一致。</p>
<ul>
<li><a href="https://github.com/tc39/proposal-string-pad-start-end" target="_blank" rel="noopener">ECMAScript spec proposal for String.prototype.{padStart,padEnd}</a></li>
</ul>
<h3 id="Object-getOwnPropertyDescriptors"><a href="#Object-getOwnPropertyDescriptors" class="headerlink" title="Object.getOwnPropertyDescriptors"></a>Object.getOwnPropertyDescriptors</h3><p>该方法是 <code>Object.getOwnPropertyDescriptor</code> 的复数形式：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">getOwnPropertyDescriptors</span>(<span class="params">obj</span>) </span>{</span><br><span class="line"> <span class="keyword">const</span> result = {};</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">let</span> key <span class="keyword">of</span> <span class="built_in">Reflect</span>.ownKeys(obj)) {</span><br><span class="line"> result[key] = <span class="built_in">Object</span>.getOwnPropertyDescriptor(obj, key);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> result;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>旨在简化对象复制的过程，它允许装饰器能够轻松的从另一个类或对象中提取出所有描述符，并将它们分配给一个新的对象。</p>
<p>由于 <code>Object.assign</code> 方法不够完美，这个是用一种吞噬行为的方式复制对象，也就是依然会出现键值覆盖的情形。</p>
<p>配合 <code>Object.create</code> 可以实现一个对象的深拷贝：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> clone = <span class="function"><span class="params">obj</span> =></span> <span class="built_in">Object</span>.create(<span class="built_in">Object</span>.getPrototypeOf(obj), <span class="built_in">Object</span>.getOwnPropertyDescriptors(obj));</span><br></pre></td></tr></table></figure>
<p>更多 <code>Object.getOwnPropertyDescriptor</code> 的使用场景可以参阅：<a href="http://2ality.com/2016/02/object-getownpropertydescriptors.html" target="_blank" rel="noopener">ES proposal: Object.getOwnPropertyDescriptors()</a>。</p>
<h3 id="函数的拖尾逗号"><a href="#函数的拖尾逗号" class="headerlink" title="函数的拖尾逗号"></a>函数的拖尾逗号</h3><p>以前的函数在参数定义时或者调用时，不允许出现拖尾逗号，如下面的代码：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params"></span></span></span><br><span class="line"><span class="function"><span class="params"> bar,</span></span></span><br><span class="line"><span class="function"><span class="params"> baz,</span></span></span><br><span class="line"><span class="function"><span class="params"></span>) </span>{ ... }</span><br><span class="line">foo(</span><br><span class="line"> param1,</span><br><span class="line"> param2,</span><br><span class="line">);</span><br></pre></td></tr></table></figure>
<p>在之前的版本中是错误的，但是新规范中，这可以的，拖尾逗号将被忽略，如同<strong>数组和对象字面量</strong>中的拖尾逗号。</p>
<h2 id="拓展阅读"><a href="#拓展阅读" class="headerlink" title="拓展阅读"></a>拓展阅读</h2><ul>
<li><a href="http://2ality.com/2016/01/ecmascript-2016.html" target="_blank" rel="noopener">The final feature set of ECMAScript 2016 (ES7)</a></li>
<li><a href="https://github.com/tc39/proposals/blob/master/README.md#active-proposals" target="_blank" rel="noopener">tc39/proposals - Active proposals</a></li>
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<div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-2"><a class="nav-link" href="#主要特性"><span class="nav-number">1.</span> <span class="nav-text">主要特性</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#异步函数"><span class="nav-number">1.1.</span> <span class="nav-text">异步函数</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#共享内存和原子"><span class="nav-number">1.2.</span> <span class="nav-text">共享内存和原子</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#次要更新"><span class="nav-number">2.</span> <span class="nav-text">次要更新</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#Object-values-Object-entries"><span class="nav-number">2.1.</span> <span class="nav-text">Object.values/Object.entries</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#字符串填充"><span class="nav-number">2.2.</span> <span class="nav-text">字符串填充</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#String-prototype-padStart"><span class="nav-number">2.2.1.</span> <span class="nav-text">String.prototype.padStart</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#String-prototype-padEnd"><span class="nav-number">2.2.2.</span> <span class="nav-text">String.prototype.padEnd</span></a></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#Object-getOwnPropertyDescriptors"><span class="nav-number">2.3.</span> <span class="nav-text">Object.getOwnPropertyDescriptors</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#函数的拖尾逗号"><span class="nav-number">2.4.</span> <span class="nav-text">函数的拖尾逗号</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#拓展阅读"><span class="nav-number">3.</span> <span class="nav-text">拓展阅读</span></a></li></ol></div>
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