JavaScript异步编程的终极演变

写在前面

有一个有趣的问题：

为什么Node.js约定回调函数的第一个参数必须是错误对象err(如果没有错误，该参数就是null)?

原因是执行回调函数对应的异步操作，它的执行分成两段，这两段之间抛出的错误程序无法捕获，所以只能作为参数传入第二段。大家知道，JavaScript只有一个线程，如果没有异步编辑，复杂的程序基本没法使用。在ES6诞生以前，异步编程的方式大概有下面四种：

• 回调函数
• 事件监听
• 发布/订阅
• Promise对象

ES6将JavaScript异步编程带入了一个全新的阶段，ES7中的async函数更是给出了异步编程的终极解决方案。下面将具体讲解异步编程的原理和值得注意的地方，待我细细道来～

异步编程的演变

基本理解

所谓异步，简单地说就是一个任务分成两段，先执行第一段，然后转而执行其他任务，等做好准备再回过头执行第二段。

举个例子 
读取一个文件进行处理，任务的第一段是向操作系统发出请求，要求读取文件。然后，程序执行其他任务，等到操作系统返回文件，再接着执行任务的第二段（处理文件）。这种不连续的执行，就叫做异步。

相应地，连续的执行就叫作同步。由于是连续执行，不能插入其他任务，所以操作系统从硬盘读取文件的这段时间，程序只能干等着。

回调函数

所谓回调函数，就是把任务的第二段单独写在一个函数中，等到重新执行该任务时直接调用这个函数。其英文名字 callback 直译过来就是 “重新调用”的意思。

拿上面的例子讲，读取文件操作是这样的：

<code class="language-JavaScript hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">fs.readFile<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(fileA, (err, data) => {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (err) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> err;
    <span class="hljs-built_in" style="box-sizing: border-box;">console</span>.log(data)
})</span>

<span class="hljs-title" style="box-sizing: border-box;">fs</span>.<span class="hljs-title" style="box-sizing: border-box;">readFile</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(fileB, (err, data) => {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (err) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> err;
    <span class="hljs-built_in" style="box-sizing: border-box;">console</span>.log(data)
})</span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

注意：上面两段代码彼此是异步的，虽然开始执行的顺序是从上到下，但是第二段并不会等到第一段结束才执行，而是并发执行。

那么问题来了，如果想fileB等到fileA读取成功后再开始执行应该怎么处理呢？最简单的办法是通过 回调嵌套：

<code class="language-JavaScript hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">fs.readFile<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(fileA, (err, data) => {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (err) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> err;
    <span class="hljs-built_in" style="box-sizing: border-box;">console</span>.log(data)

    fs.readFile(fileB, (_err, _data) => { 
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (_err) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> err;
        <span class="hljs-built_in" style="box-sizing: border-box;">console</span>.log(_data)
    })
})</span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

这种方式我只能容忍个位数字的嵌套，而且它使得代码横向发展，实在是丑的一笔，次数多了根本是没法看。试想万一要同步执行100个异步操作呢？疯掉算了吧！有没有更好的办法呢？

使用Promise

要澄清一点，Promise的概念并不是ES6新出的，而是ES6整合了一套新的写法。同样继续上面的例子，使用Promise代码就变成这样了：

<code class="language-JavaScript hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-reserved" style="box-sizing: border-box;">var</span> readFile = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">require</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'fs-readfile-promise'</span>);

readFile(fileA)
.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">then</span><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">((data)=>{<span class="hljs-built_in" style="box-sizing: border-box;">console</span>.log(data)})</span>
.<span class="hljs-title" style="box-sizing: border-box;">then</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(()=>{<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> readFile(fileB)})</span>
.<span class="hljs-title" style="box-sizing: border-box;">then</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">((data)=>{<span class="hljs-built_in" style="box-sizing: border-box;">console</span>.log(data)})</span>
// ... 读取<span class="hljs-title" style="box-sizing: border-box;">n</span>次
.<span class="hljs-title" style="box-sizing: border-box;">catch</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">((err)=>{<span class="hljs-built_in" style="box-sizing: border-box;">console</span>.log(err)})</span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

注意：上面代码使用了Node封装好的Promise版本的readFile函数，它的原理其实就是返回一个Promise对象，咱也简单地写一个：

<code class="language-JavaScript hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-reserved" style="box-sizing: border-box;">var</span> fs = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">require</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'fs'</span>);

<span class="hljs-reserved" style="box-sizing: border-box;">var</span> readFile = <span class="hljs-reserved" style="box-sizing: border-box;">function</span>(path) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Promise<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">((resolve, reject) => {
        fs.readFile(path, (err, data) => {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (err) reject(err)
            resolve(data)
        })
    })</span>
}

<span class="hljs-title" style="box-sizing: border-box;">module</span>.<span class="hljs-title" style="box-sizing: border-box;">export</span> = <span class="hljs-title" style="box-sizing: border-box;">readFile</span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

但是，Promise的写法只是回调函数的改进，使用then()之后，异步任务的两段执行看得更清楚，除此之外并无新意。撇开优点，Promise的最大问题就是代码冗余，原来的任务被Promise包装一下，不管什么操作，一眼看上去都是一堆then()，原本的语意变得很不清楚。

把酒问苍天，MD还有更好的办法吗？

使用Generator

在引入generator之前，先介绍一下什么叫 协程

“携程在手，说走就走”。哈哈，别混淆了， “协程” 非 “携程”

协程

所谓 “协程” ，就是多个线程相互协作，完成异步任务。协程有点像函数，又有点像线程。其运行流程大致如下：

• 第一步： 协程A开始执行
• 第二步：协程A执行到一半，暂停，执行权转移到协程B
• 第三步：一段时间后，协程B交还执行权
• 第四步：协程A恢复执行

<code class="language-JavaScript hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> asyncJob() {
    // <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span> 其他代码
    var f = yield readFile(fileA);
    // <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span> 其他代码 
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

上面的aysncJob()就是一个协程，它的奥妙就在于其中的yield命令。它表示执行到此处执行权交给其他协程，换而言之，yield就是异步两个阶段的分界线。

协程遇到yield命令就暂停，等到执行权返回，再从暂停的地方继续往后执行。它的最大优点就是代码的写法非常像同步操作，如果除去 yield命令，简直一模一样。

Generator函数

Generator函数是协程在ES6中的实现，最大的特点就是可以交出函数的执行权（即暂停执行）。整个Generator函数就是一个封装的异步任务，或者说就是异步任务的容器。

<code class="language-JavaScript hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span>* <span class="hljs-title" style="box-sizing: border-box;">gen</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(x)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> y = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">yield</span> x + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> y;
} 

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> g = gen(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>);
g.next() <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// { value: 3, done: false }</span>
g.next() <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// { value: undefined, done: true }</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

上面的代码中，调用Generator函数，会返回一个内部指针（即遍历器）g，这是Generator函数不同于普通函数的另一个地方，即执行它不会返回结果，返回的是指针对象。调用指针g的next()方法，会移动内部指针（即执行异步任务的第一段），指向第一个遇到的yield语句。

换而言之，next()方法的作用是分阶段执行Generator函数。每次调用next()方法，会返回一个对象，表示当前阶段的信息（value属性和done属性）。value属性是yield语句后面表达式的值，表示当前阶段的值；done属性是一个布尔值，表示Generator函数是否执行完毕，即是否还有一个阶段。

Generator函数的数据交换和错误处理

Generator函数可以暂停执行和恢复执行，这是它封装异步任务的根本原因。除此之外，它还有两个特性，使它可以作为异步编程的解决方案：函数体内外的数据交换和错误处理机制。

next()方法返回值的value属性，是Generator函数向外输出的数据；next()方法还可以接受参数，向Generator函数体内输入数据。

<code class="language-JavaScript hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span>* <span class="hljs-title" style="box-sizing: border-box;">gen</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(x)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> y = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">yield</span> x + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> y;
} 

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> g = gen(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>);
g.next()      <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// { value: 3, done: false }</span>
g.next(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)     <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// { value: 2, done: true }</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

上面的代码中，第一个next()方法的value属性，返回表达式x+2的值（3）。第二个next()方法带有参数2，这个参数可以传入Generator函数，作为上个阶段异步任务的返回结果，被函数体内的变量y接收，因此这一步的value属性返回的就是2（变量y的值）。

Generator函数内部还可以部署错误处理代码，捕获函数体外抛出的错误。

<code class="language-JavaScript hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span>* <span class="hljs-title" style="box-sizing: border-box;">gen</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(x)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> y = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">yield</span> x + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>
    } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span>(e) {
        console.log(e)
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> y
}

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> g = gen(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>);
g.next();
g.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'出错了'</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

上面代码的最后一行，Generator函数体外，使用指针对象的throw方法抛出的错误，可以被函数体内的try...catch 代码块捕获。这意味着，出错的代码与处理错误的代码，实现了时间和空间上的分离，这对于异步编程无疑是很重要的。

异步任务的封装

下面看看如何使用Generator函数，执行一个真实的异步任务。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> fetch = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">require</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'node-fetch'</span>)

<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span>* <span class="hljs-title" style="box-sizing: border-box;">gen</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> url = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'https://api.github.com/usrs/github'</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> result = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">yield</span> fetch(url);
    console.log(result.bio);
} </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

上面代码中，Generator函数封装了一个异步操作，该操作先读取一个远程接口，然后从JSON格式的数据解析信息。就像前面说过的，这段代码非常像同步操作。除了加上yield命令。

执行这段代码的方法如下：

<code class="hljs lua has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">var g = gen();
var result = g.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">next</span>();

result.value.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">then</span>(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(data)</span></span> {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> data.json()
}).<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">then</span>(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(data)</span></span> {
    g.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">next</span>(data)
});</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

上面代码中，首先执行Generator函数，获取遍历器对象。然后使用next()方法，执行异步任务的第一阶段。由于Fetch模块返回的是一个Promise对象，因此需要用then()方法调用下一个next()方法。

可以看到，虽然Generator函数将异步操作表示得很简洁，但是流程管理却不方便（即合适执行第一阶段，何时执行第二阶段）

大Boss登场之 async函数

所谓async函数，其实是Generator函数的语法糖。

继续我们异步读取文件的例子，使用Generator实现

<code class="language-JavaScript hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-reserved" style="box-sizing: border-box;">var</span> fs = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">require</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'fs'</span>);

<span class="hljs-reserved" style="box-sizing: border-box;">var</span> <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-title" style="box-sizing: border-box;">readFile</span> = <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(path)</span> =></span> {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Promise<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">((resolve, reject) => {
        fs.readFile(path, (err, data) => {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (err) reject(err)
            resolve(data)
        })
    })</span>
}

<span class="hljs-title" style="box-sizing: border-box;">var</span> <span class="hljs-title" style="box-sizing: border-box;">gen</span> = <span class="hljs-title" style="box-sizing: border-box;">function</span>* <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {
    <span class="hljs-title" style="box-sizing: border-box;">var</span> <span class="hljs-title" style="box-sizing: border-box;">f1</span> = <span class="hljs-title" style="box-sizing: border-box;">yield</span> <span class="hljs-title" style="box-sizing: border-box;">readFile</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(fileA)</span>;
    <span class="hljs-title" style="box-sizing: border-box;">var</span> <span class="hljs-title" style="box-sizing: border-box;">f2</span> = <span class="hljs-title" style="box-sizing: border-box;">yield</span> <span class="hljs-title" style="box-sizing: border-box;">readFile</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(fileB)</span>;
    <span class="hljs-title" style="box-sizing: border-box;">console</span>.<span class="hljs-title" style="box-sizing: border-box;">log</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(f1.toString())</span>;
    <span class="hljs-title" style="box-sizing: border-box;">console</span>.<span class="hljs-title" style="box-sizing: border-box;">log</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(f2.toString())</span>;
}</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

写成async函数，就是下面这样：

<code class="language-JavaScript hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> asyncReadFile = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">async</span> function() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> f1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">await</span> readFile(fileA);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> f2 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">await</span> readFile(fileB);
    console.log(f1.toString())
    console.log(f2.toString())
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

发现了吧，async函数就是将Generator函数的*替换成了async，将yield替换成await，除此之外，还对 Generator做了以下四点改进：

（1）内置执行器。Generator函数的执行比如靠执行器，所以才有了co模块等异步执行器，而async函数是自带执行器的。也就是说：async函数的执行，与普通函数一模一样，只要一行：

<code class="language-JavaScript hljs oxygene has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">result</span> = asyncReadFile();</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

（2）上面的代码调用了asyncReadFile()，就会自动执行，输出最后结果。这完全不像Generator函数，需要调用next()方法，或者使用co模块，才能得到真正执行，从而得到最终结果。

（3）更好的语义。async和await比起星号和yield，语义更清楚。async表示函数里有异步操作，await表示紧跟在后面的表达式需要等待结果。

（4）更广的适用性。async函数的await命令后面可以是Promise对象和原始类型的值（数值、字符串和布尔值，而这是等同于同步操作）。

（5）返回值是Promise，这比Generator函数返回的是Iterator对象方便多了。你可以用then()指定下一步操作。

进一步说，async函数完全可以看作由多个异步操作包装成的一个Promise对象，而await命令就是内部then()命令的语法糖。

实现原理

async函数的实现就是将Generator函数和自动执行器包装在一个函数中。如下代码：

<code class="language-JavaScript hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">async <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">fn</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(args)</span> {</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// ...</span>
}

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 等同于 </span>
<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">fn</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(args)</span> {</span>
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> spawn(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span>*<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// ...</span>
  })
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 自动执行器</span>
<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">spawn</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(genF)</span> {</span>
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Promise(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(resolve, reject)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> gen = genF();
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">step</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(nextF)</span> {</span>
      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> next = nextF()
      } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span>(e) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> reject(e)
      }
      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (next.done) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> resolve(next.value)
      }
      Promise.resolve(next.value).then(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(v)</span> {</span>
        step(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gen.next(v) })
      },<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(e)</span> {</span>
        step(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gen.throw(e) })
      })
    }
    step(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gen.next(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">undefined</span>) })
  })
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul>

async函数用法

（1）async函数返回一个Promise对象，可以是then()方法添加回调函数。 
（2）当函数执行时，一旦遇到await()就会先返回，等到触发的异步操作完成，再接着执行函数体内后面的语句。

下面是一个延迟输出结果的例子：

<code class="language-JavaScript hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-reserved" style="box-sizing: border-box;">function</span> timeout(ms) {
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Promise<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">((resolve) => {
    setTimeout(resolve, ms)
  })</span>
}

<span class="hljs-title" style="box-sizing: border-box;">async</span> <span class="hljs-title" style="box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">asyncPrint</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(value, ms)</span> {
  <span class="hljs-title" style="box-sizing: border-box;">await</span> <span class="hljs-title" style="box-sizing: border-box;">timeout</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(ms)</span>
  <span class="hljs-title" style="box-sizing: border-box;">console</span>.<span class="hljs-title" style="box-sizing: border-box;">log</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(value)</span>
}

// 延迟500<span class="hljs-title" style="box-sizing: border-box;">ms</span>后输出 "<span class="hljs-title" style="box-sizing: border-box;">Hello</span> <span class="hljs-title" style="box-sizing: border-box;">World</span>!"
<span class="hljs-title" style="box-sizing: border-box;">asyncPrint</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Hello World!'</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">500</span>)</span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

注意事项

（1）await命令后面的Promise对象，运行结果可能是reject，所以最好把await命令放在try...catch代码块中。

（2）await命令只能用在async函数中，用在普通函数中会报错。

（3）ES6将await增加为保留字。如果使用这个词作为标识符，在ES5中是合法的，但是ES6会抛出 SyntaxError（语法错误）。

终极一战

“倚天不出谁与争锋”，上面介绍了一大堆，最后还是让我们通过一个例子来看看 async 函数和Promise、Generator到底谁才是真正的老大吧！

需求：假定某个DOM元素上部署了一系列的动画，前一个动画结束才能开始后一个。如果当中又一个动画出错就不再往下执行，返回上一个成功执行动画的返回值。

用Promise实现

<code class="language-JavaScript hljs actionscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">chainAnimationsPromise</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(ele, animations)</span> {</span>

  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 变量ret用来保存上一个动画的返回值 </span>
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> ret = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>;

  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 新建一个空的Promise </span>
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> p = Promise.resolve();

  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 使用then方法添加所有动画 </span>
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> anim <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> animations) {
    p = p.then(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(val)</span> {</span>
      ret = val;
      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> anim(ele);
    })
  }

  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 返回一个部署了错误捕获机制的Promise </span>
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> p.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span>(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(e)</span> {</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* 忽略错误，继续执行 */</span>
  }).then(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> ret;
  })
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul>

虽然Promise的写法比起回调函数的写法有很大的改进，但是操作本身的语义却变得不太明朗。

用Generator实现

<code class="language-JavaScript hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">chainAnimationsGenerator</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(ele, animations)</span> {</span>
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> spawn(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span>*<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> ret = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> anim of animations) {
        ret = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">yield</span> anim(ele)
      }
    } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span>(e) {
      <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* 忽略错误，继续执行 */</span>
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> ret;
  })
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

使用Generator虽然语义比Promise写法清晰不少，但是用户定义的操作全部出现在spawn函数的内部。这个写法的问题在于，必须有一个任务运行器自动执行Generator函数，它返回一个Promise对象，而且保证yield语句后的表达式返回的是一个Promise。上面的spawn就扮演了这一角色。它的实现如下：

<code class="language-JavaScript hljs lua has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">spawn</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(genF)</span></span> {
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> new Promise(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(resolve, reject)</span></span> {
    var gen = genF();
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">step</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(nextF)</span></span> {
      try {
        var <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">next</span> = nextF()
      } catch(e) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> reject(e)
      }
      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">next</span>.done) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> resolve(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">next</span>.value)
      }
      Promise.resolve(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">next</span>.value).<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">then</span>(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(v)</span></span> {
        step(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span></span> { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gen.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">next</span>(v) })
      },<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(e)</span></span> {
        step(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span></span> { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gen.throw(e) })
      })
    }
    step(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span></span> { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gen.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">next</span>(undefined) })
  })
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul>

使用async实现

<code class="language-JavaScript hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">async <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">chainAnimationAsync</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(ele, animations)</span> {</span>
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> ret = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>;
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> anim of animations) {
      ret = await anim(ele)
    } 
  } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span>(e) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* 忽略错误，继续执行 */</span>
  }
  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> ret;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

好了，光从代码量上就看出优势了吧！简洁又符合语义，几乎没有不相关代码。完胜！

注意一点：async属于ES7的提案，使用时请通过babel或者regenerator进行转码。

参考

阮一峰 《ES6标准入门》