【React系列】ES6学习笔记(二)Promise、async\await

本文参考自电子书《ECMAScript 6 入门》:https://es6.ruanyifeng.com/

Promise 对象

1. Promise 的含义

Promise 是异步编程的一种解决方案,比传统的解决方案——回调函数和事件——更合理和更强大。它由社区最早提出和实现,ES6 将其写进了语言标准,统一了用法,原生提供了 Promise 对象。

所谓 Promise,简单说就是一个容器,里面保存着某个未来才会结束的事件(通常是一个异步操作)的结果。从语法上说,Promise 是一个对象,从它可以获取异步操作的消息。Promise 提供统一的 API,各种异步操作都可以用同样的方法进行处理。

Promise 对象有以下两个特点。

  1. 对象的状态不受外界影响。Promise 对象代表一个异步操作,有三种状态:pending(进行中)、fulfilled(已成功)和rejected(已失败)。只有异步操作的结果,可以决定当前是哪一种状态,任何其他操作都无法改变这个状态。这也是 Promise 这个名字的由来,它的英语意思就是“承诺”,表示其他手段无法改变。

  2. 一旦状态改变,就不会再变,任何时候都可以得到这个结果。Promise 对象的状态改变,只有两种可能:从 pending 变为 fulfilled 和从 pending 变为 rejected。只要这两种情况发生,状态就凝固了,不会再变了,会一直保持这个结果,这时就称为 resolved(已定型)。如果改变已经发生了,你再对 Promise 对象添加回调函数,也会立即得到这个结果。这与事件(Event)完全不同,事件的特点是,如果你错过了它,再去监听,是得不到结果的。

注意,为了行文方便,本章后面的 resolved 统一只指 fulfilled 状态,不包含 rejected 状态。

有了 Promise 对象,就可以将异步操作以同步操作的流程表达出来,避免了层层嵌套的回调函数。此外,Promise 对象提供统一的接口,使得控制异步操作更加容易。

Promise 也有一些缺点。首先,无法取消 Promise,一旦新建它就会立即执行,无法中途取消。其次,如果不设置回调函数,Promise 内部抛出的错误,不会反应到外部。第三,当处于 pending 状态时,无法得知目前进展到哪一个阶段(刚刚开始还是即将完成)。

如果某些事件不断地反复发生,一般来说,使用 Stream 模式是比部署 Promise 更好的选择。

2. 基本用法

ES6 规定,Promise对象是一个构造函数,用来生成Promise实例。

下面代码创造了一个Promise实例。

const promise = new Promise(function(resolve, reject) {
  // ... some code

  if (/* 异步操作成功 */){
    resolve(value);
  } else {
    reject(error);
  }
});

Promise 构造函数接受一个函数作为参数,该函数的两个参数分别是 resolvereject。它们是两个函数,由 JavaScript 引擎提供,不用自己部署。

resolve 函数的作用是,将 Promise 对象的状态从“未完成”变为“成功”(即从 pending 变为 resolved),在异步操作成功时调用,并将异步操作的结果,作为参数传递出去;reject 函数的作用是,将 Promise 对象的状态从“未完成”变为“失败”(即从 pending 变为 rejected),在异步操作失败时调用,并将异步操作报出的错误,作为参数传递出去。

Promise 实例生成以后,可以用 then 方法分别指定 resolved 状态和 rejected 状态的回调函数。

promise.then(function(value) {
  // success
}, function(error) {
  // failure
});

then 方法可以接受两个回调函数作为参数。第一个回调函数是 Promise 对象的状态变为 resolved 时调用,第二个回调函数是 Promise 对象的状态变为 rejected 时调用。这两个函数都是可选的,不一定要提供。它们都接受 Promise 对象传出的值作为参数。

下面是一个 Promise 对象的简单例子。

function timeout(ms) {
  return new Promise((resolve, reject) => {
    setTimeout(resolve, ms, 'done');
  });
}

timeout(100).then((value) => {
  console.log(value);
});

上面代码中,timeout 方法返回一个 Promise 实例,表示一段时间以后才会发生的结果。过了指定的时间(ms 参数)以后,Promise 实例的状态变为 resolved,就会触发 then 方法绑定的回调函数。

Promise 新建后就会立即执行。

let promise = new Promise(function(resolve, reject) {
  console.log('Promise');
  resolve();
});

promise.then(function() {
  console.log('resolved.');
});

console.log('Hi!');

// Promise
// Hi!
// resolved

上面代码中,Promise 新建后立即执行,所以首先输出的是 Promise。然后,then 方法指定的回调函数,将在当前脚本所有同步任务执行完才会执行,所以 resolved 最后输出。

下面是异步加载图片的例子。

function loadImageAsync(url) {
  return new Promise(function(resolve, reject) {
    const image = new Image();

    image.onload = function() {
      resolve(image);
    };

    image.onerror = function() {
      reject(new Error('Could not load image at ' + url));
    };

    image.src = url;
  });
}

上面代码中,使用Promise包装了一个图片加载的异步操作。如果加载成功,就调用resolve方法,否则就调用reject方法。

下面是一个用Promise对象实现的 Ajax 操作的例子。

const getJSON = function(url) {
  const promise = new Promise(function(resolve, reject){
    const handler = function() {
      if (this.readyState !== 4) {
        return;
      }
      if (this.status === 200) {
        resolve(this.response);
      } else {
        reject(new Error(this.statusText));
      }
    };
    const client = new XMLHttpRequest();
    client.open("GET", url);
    client.onreadystatechange = handler;
    client.responseType = "json";
    client.setRequestHeader("Accept", "application/json");
    client.send();

  });

  return promise;
};

getJSON("/posts.json").then(function(json) {
  console.log('Contents: ' + json);
}, function(error) {
  console.error('出错了', error);
});

上面代码中,getJSON 是对 XMLHttpRequest 对象的封装,用于发出一个针对 JSON 数据的 HTTP 请求,并且返回一个 Promise 对象。需要注意的是,在 getJSON 内部,resolve 函数和 reject 函数调用时,都带有参数。

如果调用 resolve 函数和 reject 函数时带有参数,那么它们的参数会被传递给回调函数。reject 函数的参数通常是 Error 对象的实例,表示抛出的错误;resolve 函数的参数除了正常的值以外,还可能是另一个 Promise 实例,比如像下面这样。

const p1 = new Promise(function (resolve, reject) {
  // ...
});

const p2 = new Promise(function (resolve, reject) {
  // ...
  resolve(p1);
})

上面代码中,p1p2 都是 Promise 的实例,但是 p2resolve 方法将 p1 作为参数,即一个异步操作的结果是返回另一个异步操作。

注意,这时 p1 的状态就会传递给 p2,也就是说,p1 的状态决定了 p2 的状态。如果 p1 的状态是 pending,那么 p2 的回调函数就会等待 p1 的状态改变;如果 p1 的状态已经是 resolved 或者 rejected,那么 p2 的回调函数将会立刻执行。

const p1 = new Promise(function (resolve, reject) {
  setTimeout(() => reject(new Error('fail')), 3000)
})

const p2 = new Promise(function (resolve, reject) {
  setTimeout(() => resolve(p1), 1000)
})

p2
  .then(result => console.log(result))
  .catch(error => console.log(error))
// Error: fail

上面代码中,p1 是一个 Promise,3 秒之后变为 rejectedp2 的状态在 1 秒之后改变,resolve 方法返回的是 p1。由于 p2 返回的是另一个 Promise,导致 p2 自己的状态无效了,由 p1 的状态决定 p2 的状态。所以,后面的 then 语句都变成针对后者(p1)。又过了 2 秒,p1 变为 rejected,导致触发 catch 方法指定的回调函数。

注意,调用 resolvereject 并不会终结 Promise 的参数函数的执行。

new Promise((resolve, reject) => {
  resolve(1);
  console.log(2);
}).then(r => {
  console.log(r);
});
// 2
// 1

上面代码中,调用 resolve(1) 以后,后面的 console.log(2) 还是会执行,并且会首先打印出来。这是因为立即 resolved 的 Promise 是在本轮事件循环的末尾执行,总是晚于本轮循环的同步任务。

一般来说,调用 resolvereject 以后,Promise 的使命就完成了,后继操作应该放到 then 方法里面,而不应该直接写在 resolvereject 的后面。所以,最好在它们前面加上 return 语句,这样就不会有意外。

new Promise((resolve, reject) => {
  return resolve(1);
  // 后面的语句不会执行
  console.log(2);
})

3. Promise.prototype.then()

Promise 实例具有 then 方法,也就是说,then 方法是定义在原型对象 Promise.prototype 上的。它的作用是为 Promise 实例添加状态改变时的回调函数。前面说过,then 方法的第一个参数是 resolved 状态的回调函数,第二个参数是 rejected 状态的回调函数,它们都是可选的。

then 方法返回的是一个新的 Promise 实例(注意,不是原来那个 Promise 实例)。因此可以采用链式写法,即 then 方法后面再调用另一个 then 方法。

getJSON("/posts.json").then(function(json) {
  return json.post;
}).then(function(post) {
  // ...
});

上面的代码使用 then 方法,依次指定了两个回调函数。第一个回调函数完成以后,会将返回结果作为参数,传入第二个回调函数。

采用链式的 then,可以指定一组按照次序调用的回调函数。这时,前一个回调函数,有可能返回的还是一个 Promise 对象(即有异步操作),这时后一个回调函数,就会等待该 Promise 对象的状态发生变化,才会被调用。

getJSON("/post/1.json").then(function(post) {
  return getJSON(post.commentURL);
}).then(function (comments) {
  console.log("resolved: ", comments);
}, function (err){
  console.log("rejected: ", err);
});

上面代码中,第一个 then 方法指定的回调函数,返回的是另一个 Promise 对象。这时,第二个 then 方法指定的回调函数,就会等待这个新的 Promise 对象状态发生变化。如果变为 resolved,就调用第一个回调函数,如果状态变为 rejected,就调用第二个回调函数。

如果采用箭头函数,上面的代码可以写得更简洁。

getJSON("/post/1.json").then(
  post => getJSON(post.commentURL)
).then(
  comments => console.log("resolved: ", comments),
  err => console.log("rejected: ", err)
);

4. Promise.prototype.catch()

Promise.prototype.catch() 方法是 .then(null, rejection).then(undefined, rejection) 的别名,用于指定发生错误时的回调函数。

getJSON('/posts.json').then(function(posts) {
  // ...
}).catch(function(error) {
  // 处理 getJSON 和 前一个回调函数运行时发生的错误
  console.log('发生错误!', error);
});

上面代码中,getJSON() 方法返回一个 Promise 对象,如果该对象状态变为 resolved,则会调用 then() 方法指定的回调函数;如果异步操作抛出错误,状态就会变为 rejected,就会调用 catch() 方法指定的回调函数,处理这个错误。另外,then() 方法指定的回调函数,如果运行中抛出错误,也会被 catch() 方法捕获。

p.then((val) => console.log('fulfilled:', val))
  .catch((err) => console.log('rejected', err));

// 等同于
p.then((val) => console.log('fulfilled:', val))
  .then(null, (err) => console.log("rejected:", err));

下面是一个例子。

const promise = new Promise(function(resolve, reject) {
  throw new Error('test');
});
promise.catch(function(error) {
  console.log(error);
});
// Error: test

上面代码中,promise抛出一个错误,就被catch()方法指定的回调函数捕获。注意,上面的写法与下面两种写法是等价的。

// 写法一
const promise = new Promise(function(resolve, reject) {
  try {
    throw new Error('test');
  } catch(e) {
    reject(e);
  }
});
promise.catch(function(error) {
  console.log(error);
});

// 写法二
const promise = new Promise(function(resolve, reject) {
  reject(new Error('test'));
});
promise.catch(function(error) {
  console.log(error);
});

比较上面两种写法,可以发现reject()方法的作用,等同于抛出错误。

如果 Promise 状态已经变成resolved,再抛出错误是无效的。

const promise = new Promise(function(resolve, reject) {
  resolve('ok');
  throw new Error('test');
});
promise
  .then(function(value) { console.log(value) })
  .catch(function(error) { console.log(error) });
// ok

上面代码中,Promise 在resolve语句后面,再抛出错误,不会被捕获,等于没有抛出。因为 Promise 的状态一旦改变,就永久保持该状态,不会再变了。

Promise 对象的错误具有“冒泡”性质,会一直向后传递,直到被捕获为止。也就是说,错误总是会被下一个catch语句捕获。

getJSON('/post/1.json').then(function(post) {
  return getJSON(post.commentURL);
}).then(function(comments) {
  // some code
}).catch(function(error) {
  // 处理前面三个Promise产生的错误
});

上面代码中,一共有三个 Promise 对象:一个由 getJSON() 产生,两个由 then() 产生。它们之中任何一个抛出的错误,都会被最后一个 catch() 捕获。

一般来说,不要在 then() 方法里面定义 Reject 状态的回调函数(即 then 的第二个参数),总是使用 catch 方法。

// bad
promise
  .then(function(data) {
    // success
  }, function(err) {
    // error
  });

// good
promise
  .then(function(data) { //cb
    // success
  })
  .catch(function(err) {
    // error
  });

上面代码中,第二种写法要好于第一种写法,理由是第二种写法可以捕获前面 then 方法执行中的错误,也更接近同步的写法(try/catch)。因此,建议总是使用 catch() 方法,而不使用 then() 方法的第二个参数。

跟传统的 try/catch 代码块不同的是,如果没有使用 catch() 方法指定错误处理的回调函数,Promise 对象抛出的错误不会传递到外层代码,即不会有任何反应。

const someAsyncThing = function() {
  return new Promise(function(resolve, reject) {
    // 下面一行会报错,因为x没有声明
    resolve(x + 2);
  });
};

someAsyncThing().then(function() {
  console.log('everything is great');
});

setTimeout(() => { console.log(123) }, 2000);
// Uncaught (in promise) ReferenceError: x is not defined
// 123

上面代码中,someAsyncThing() 函数产生的 Promise 对象,内部有语法错误。浏览器运行到这一行,会打印出错误提示 ReferenceError: x is not defined,但是不会退出进程、终止脚本执行,2 秒之后还是会输出 123。这就是说,Promise 内部的错误不会影响到 Promise 外部的代码,通俗的说法就是“Promise 会吃掉错误”。

这个脚本放在服务器执行,退出码就是 0(即表示执行成功)。不过,Node.js 有一个 unhandledRejection 事件,专门监听未捕获的 reject 错误,上面的脚本会触发这个事件的监听函数,可以在监听函数里面抛出错误。

process.on('unhandledRejection', function (err, p) {
  throw err;
});

上面代码中,unhandledRejection 事件的监听函数有两个参数,第一个是错误对象,第二个是报错的 Promise 实例,它可以用来了解发生错误的环境信息。

注意,Node 有计划在未来废除 unhandledRejection 事件。如果 Promise 内部有未捕获的错误,会直接终止进程,并且进程的退出码不为 0。

再看下面的例子。

const promise = new Promise(function (resolve, reject) {
  resolve('ok');
  setTimeout(function () { throw new Error('test') }, 0)
});
promise.then(function (value) { console.log(value) });
// ok
// Uncaught Error: test

上面代码中,Promise 指定在下一轮“事件循环”再抛出错误。到了那个时候,Promise 的运行已经结束了,所以这个错误是在 Promise 函数体外抛出的,会冒泡到最外层,成了未捕获的错误。

一般总是建议,Promise 对象后面要跟catch()方法,这样可以处理 Promise 内部发生的错误。catch()方法返回的还是一个 Promise 对象,因此后面还可以接着调用then()方法。

const someAsyncThing = function() {
  return new Promise(function(resolve, reject) {
    // 下面一行会报错,因为x没有声明
    resolve(x + 2);
  });
};

someAsyncThing()
.catch(function(error) {
  console.log('oh no', error);
})
.then(function() {
  console.log('carry on');
});
// oh no [ReferenceError: x is not defined]
// carry on

上面代码运行完catch()方法指定的回调函数,会接着运行后面那个then()方法指定的回调函数。如果没有报错,则会跳过catch()方法。

Promise.resolve()
.catch(function(error) {
  console.log('oh no', error);
})
.then(function() {
  console.log('carry on');
});
// carry on

上面的代码因为没有报错,跳过了 catch() 方法,直接执行后面的 then() 方法。此时,要是 then() 方法里面报错,就与前面的 catch() 无关了。

catch() 方法之中,还能再抛出错误。

const someAsyncThing = function() {
  return new Promise(function(resolve, reject) {
    // 下面一行会报错,因为x没有声明
    resolve(x + 2);
  });
};

someAsyncThing().then(function() {
  return someOtherAsyncThing();
}).catch(function(error) {
  console.log('oh no', error);
  // 下面一行会报错,因为 y 没有声明
  y + 2;
}).then(function() {
  console.log('carry on');
});
// oh no [ReferenceError: x is not defined]

上面代码中,catch()方法抛出一个错误,因为后面没有别的catch()方法了,导致这个错误不会被捕获,也不会传递到外层。如果改写一下,结果就不一样了。

someAsyncThing().then(function() {
  return someOtherAsyncThing();
}).catch(function(error) {
  console.log('oh no', error);
  // 下面一行会报错,因为y没有声明
  y + 2;
}).catch(function(error) {
  console.log('carry on', error);
});
// oh no [ReferenceError: x is not defined]
// carry on [ReferenceError: y is not defined]

上面代码中,第二个catch()方法用来捕获前一个catch()方法抛出的错误。

5. Promise.prototype.finally()

finally()方法用于指定不管 Promise 对象最后状态如何,都会执行的操作。该方法是 ES2018 引入标准的。

promise
.then(result => {···})
.catch(error => {···})
.finally(() => {···});

上面代码中,不管 promise 最后的状态,在执行完 thencatch 指定的回调函数以后,都会执行 finally 方法指定的回调函数。

下面是一个例子,服务器使用 Promise 处理请求,然后使用 finally 方法关掉服务器。

server.listen(port)
  .then(function () {
    // ...
  })
  .finally(server.stop);

finally 方法的回调函数不接受任何参数,这意味着没有办法知道,前面的 Promise 状态到底是 fulfilled 还是 rejected。这表明,finally 方法里面的操作,应该是与状态无关的,不依赖于 Promise 的执行结果。

finally 本质上是 then 方法的特例。

promise
.finally(() => {
  // 语句
});

// 等同于
promise
.then(
  result => {
    // 语句
    return result;
  },
  error => {
    // 语句
    throw error;
  }
);

上面代码中,如果不使用finally方法,同样的语句需要为成功和失败两种情况各写一次。有了finally方法,则只需要写一次。

它的实现也很简单。

Promise.prototype.finally = function (callback) {
  let P = this.constructor;
  return this.then(
    value  => P.resolve(callback()).then(() => value),
    reason => P.resolve(callback()).then(() => { throw reason })
  );
};

上面代码中,不管前面的 Promise 是fulfilled还是rejected,都会执行回调函数callback

从上面的实现还可以看到,finally方法总是会返回原来的值。

// resolve 的值是 undefined
Promise.resolve(2).then(() => {}, () => {})

// resolve 的值是 2
Promise.resolve(2).finally(() => {})

// reject 的值是 undefined
Promise.reject(3).then(() => {}, () => {})

// reject 的值是 3
Promise.reject(3).finally(() => {})

6. Promise.all()

Promise.all()方法用于将多个 Promise 实例,包装成一个新的 Promise 实例。

const p = Promise.all([p1, p2, p3]);

上面代码中,Promise.all() 方法接受一个数组作为参数,p1p2p3 都是 Promise 实例,如果不是,就会先调用下面讲到的 Promise.resolve 方法,将参数转为 Promise 实例,再进一步处理。另外,Promise.all() 方法的参数可以不是数组,但必须具有 Iterator 接口,且返回的每个成员都是 Promise 实例。

p 的状态由 p1p2p3 决定,分成两种情况。

  1. 只有 p1p2p3 的状态都变成 fulfilledp 的状态才会变成 fulfilled,此时 p1p2p3 的返回值组成一个数组,传递给 p 的回调函数。

  2. 只要 p1p2p3 之中有一个被 rejectedp 的状态就变成 rejected,此时第一个被 reject 的实例的返回值,会传递给 p 的回调函数。

下面是一个具体的例子。

// 生成一个Promise对象的数组
const promises = [2, 3, 5, 7, 11, 13].map(function (id) {
  return getJSON('/post/' + id + ".json");
});

Promise.all(promises).then(function (posts) {
  // ...
}).catch(function(reason){
  // ...
});

上面代码中,promises是包含 6Promise 实例的数组,只有这 6 个实例的状态都变成fulfilled,或者其中有一个变为rejected,才会调用Promise.all方法后面的回调函数。

下面是另一个例子。

const databasePromise = connectDatabase();

const booksPromise = databasePromise
  .then(findAllBooks);

const userPromise = databasePromise
  .then(getCurrentUser);

Promise.all([
  booksPromise,
  userPromise
])
.then(([books, user]) => pickTopRecommendations(books, user));

上面代码中,booksPromiseuserPromise是两个异步操作,只有等到它们的结果都返回了,才会触发pickTopRecommendations这个回调函数。

注意,如果作为参数的 Promise 实例,自己定义了catch方法,那么它一旦被rejected,并不会触发Promise.all()catch方法。

const p1 = new Promise((resolve, reject) => {
  resolve('hello');
})
.then(result => result)
.catch(e => e);

const p2 = new Promise((resolve, reject) => {
  throw new Error('报错了');
})
.then(result => result)
.catch(e => e);

Promise.all([p1, p2])
.then(result => console.log(result))
.catch(e => console.log(e));
// ["hello", Error: 报错了]

上面代码中,p1resolvedp2首先会rejected,但是p2有自己的catch方法,该方法返回的是一个新的 Promise 实例,p2指向的实际上是这个实例。该实例执行完catch方法后,也会变成resolved,导致Promise.all()方法参数里面的两个实例都会resolved,因此会调用then方法指定的回调函数,而不会调用catch方法指定的回调函数。

如果p2没有自己的catch方法,就会调用Promise.all()catch方法。

const p1 = new Promise((resolve, reject) => {
  resolve('hello');
})
.then(result => result);

const p2 = new Promise((resolve, reject) => {
  throw new Error('报错了');
})
.then(result => result);

Promise.all([p1, p2])
.then(result => console.log(result))
.catch(e => console.log(e));
// Error: 报错了

7. Promise.race()

Promise.race()方法同样是将多个 Promise 实例,包装成一个新的 Promise 实例。

const p = Promise.race([p1, p2, p3]);

上面代码中,只要p1p2p3之中有一个实例率先改变状态,p的状态就跟着改变。那个率先改变的 Promise 实例的返回值,就传递给p的回调函数。

Promise.race()方法的参数与Promise.all()方法一样,如果不是 Promise 实例,就会先调用下面讲到的Promise.resolve()方法,将参数转为 Promise 实例,再进一步处理。

下面是一个例子,如果指定时间内没有获得结果,就将 Promise 的状态变为reject,否则变为resolve

const p = Promise.race([
  fetch('/resource-that-may-take-a-while'),
  new Promise(function (resolve, reject) {
    setTimeout(() => reject(new Error('request timeout')), 5000)
  })
]);

p
.then(console.log)
.catch(console.error);

上面代码中,如果 5 秒之内fetch方法无法返回结果,变量p的状态就会变为rejected,从而触发catch方法指定的回调函数。

8. Promise.allSettled()

有时候,我们希望等到一组异步操作都结束了,不管每一个操作是成功还是失败,再进行下一步操作。但是,现有的 Promise 方法很难实现这个要求。

Promise.all()方法只适合所有异步操作都成功的情况,如果有一个操作失败,就无法满足要求。

const urls = [url_1, url_2, url_3];
const requests = urls.map(x => fetch(x));

try {
  await Promise.all(requests);
  console.log('所有请求都成功。');
} catch {
  console.log('至少一个请求失败,其他请求可能还没结束。');
}

上面示例中,Promise.all()可以确定所有请求都成功了,但是只要有一个请求失败,它就会报错,而不管另外的请求是否结束。

为了解决这个问题,ES2020 引入了Promise.allSettled()方法,用来确定一组异步操作是否都结束了(不管成功或失败)。所以,它的名字叫做”Settled“,包含了”fulfilled“和”rejected“两种情况。

Promise.allSettled()方法接受一个数组作为参数,数组的每个成员都是一个 Promise 对象,并返回一个新的 Promise 对象。只有等到参数数组的所有 Promise 对象都发生状态变更(不管是fulfilled还是rejected),返回的 Promise 对象才会发生状态变更。

const promises = [
  fetch('/api-1'),
  fetch('/api-2'),
  fetch('/api-3'),
];

await Promise.allSettled(promises);
removeLoadingIndicator();

上面示例中,数组promises包含了三个请求,只有等到这三个请求都结束了(不管请求成功还是失败),removeLoadingIndicator()才会执行。

该方法返回的新的 Promise 实例,一旦发生状态变更,状态总是fulfilled,不会变成rejected。状态变成fulfilled后,它的回调函数会接收到一个数组作为参数,该数组的每个成员对应前面数组的每个 Promise 对象。

const resolved = Promise.resolve(42);
const rejected = Promise.reject(-1);

const allSettledPromise = Promise.allSettled([resolved, rejected]);

allSettledPromise.then(function (results) {
  console.log(results);
});
// [
//    { status: 'fulfilled', value: 42 },
//    { status: 'rejected', reason: -1 }
// ]

上面代码中,Promise.allSettled()的返回值allSettledPromise,状态只可能变成fulfilled。它的回调函数接收到的参数是数组results。该数组的每个成员都是一个对象,对应传入Promise.allSettled()的数组里面的两个 Promise 对象。

results的每个成员是一个对象,对象的格式是固定的,对应异步操作的结果。

// 异步操作成功时
{status: 'fulfilled', value: value}

// 异步操作失败时
{status: 'rejected', reason: reason}

成员对象的status属性的值只可能是字符串fulfilled或字符串rejected,用来区分异步操作是成功还是失败。如果是成功(fulfilled),对象会有value属性,如果是失败(rejected),会有reason属性,对应两种状态时前面异步操作的返回值。

下面是返回值的用法例子。

const promises = [ fetch('index.html'), fetch('https://does-not-exist/') ];
const results = await Promise.allSettled(promises);

// 过滤出成功的请求
const successfulPromises = results.filter(p => p.status === 'fulfilled');

// 过滤出失败的请求,并输出原因
const errors = results
  .filter(p => p.status === 'rejected')
  .map(p => p.reason);

9. Promise.any()

ES2021 引入了Promise.any()方法。该方法接受一组 Promise 实例作为参数,包装成一个新的 Promise 实例返回。

Promise.any([
  fetch('https://v8.dev/').then(() => 'home'),
  fetch('https://v8.dev/blog').then(() => 'blog'),
  fetch('https://v8.dev/docs').then(() => 'docs')
]).then((first) => {  // 只要有一个 fetch() 请求成功
  console.log(first);
}).catch((error) => { // 所有三个 fetch() 全部请求失败
  console.log(error);
});

只要参数实例有一个变成fulfilled状态,包装实例就会变成fulfilled状态;如果所有参数实例都变成rejected状态,包装实例就会变成rejected状态。

Promise.any()Promise.race()方法很像,只有一点不同,就是Promise.any()不会因为某个 Promise 变成rejected状态而结束,必须等到所有参数 Promise 变成rejected状态才会结束。

下面是Promise()await命令结合使用的例子。

const promises = [
  fetch('/endpoint-a').then(() => 'a'),
  fetch('/endpoint-b').then(() => 'b'),
  fetch('/endpoint-c').then(() => 'c'),
];

try {
  const first = await Promise.any(promises);
  console.log(first);
} catch (error) {
  console.log(error);

上面代码中,Promise.any()方法的参数数组包含三个 Promise 操作。其中只要有一个变成fulfilledPromise.any()返回的 Promise 对象就变成fulfilled。如果所有三个操作都变成rejected,那么await命令就会抛出错误。

Promise.any()抛出的错误是一个 AggregateError 实例(详见《对象的扩展》一章),这个 AggregateError 实例对象的errors属性是一个数组,包含了所有成员的错误。

下面是一个例子。

var resolved = Promise.resolve(42);
var rejected = Promise.reject(-1);
var alsoRejected = Promise.reject(Infinity);

Promise.any([resolved, rejected, alsoRejected]).then(function (result) {
  console.log(result); // 42
});

Promise.any([rejected, alsoRejected]).catch(function (results) {
  console.log(results instanceof AggregateError); // true
  console.log(results.errors); // [-1, Infinity]
});

10. Promise.resolve()

有时需要将现有对象转为 Promise 对象,Promise.resolve()方法就起到这个作用。

const jsPromise = Promise.resolve($.ajax('/whatever.json'));

上面代码将 jQuery 生成的deferred对象,转为一个新的 Promise 对象。

Promise.resolve()等价于下面的写法。

Promise.resolve('foo')
// 等价于
new Promise(resolve => resolve('foo'))

Promise.resolve()方法的参数分成四种情况。

(1)参数是一个 Promise 实例

如果参数是 Promise 实例,那么Promise.resolve将不做任何修改、原封不动地返回这个实例。

(2)参数是一个 thenable 对象

thenable对象指的是具有then方法的对象,比如下面这个对象。

let thenable = {
  then: function(resolve, reject) {
    resolve(42);
  }
};

Promise.resolve()方法会将这个对象转为 Promise 对象,然后就立即执行thenable对象的then()方法。

let thenable = {
  then: function(resolve, reject) {
    resolve(42);
  }
};

let p1 = Promise.resolve(thenable);
p1.then(function (value) {
  console.log(value);  // 42
});

上面代码中,thenable对象的then()方法执行后,对象p1的状态就变为resolved,从而立即执行最后那个then()方法指定的回调函数,输出42

(3)参数不是具有 then() 方法的对象,或根本就不是对象

如果参数是一个原始值,或者是一个不具有then()方法的对象,则Promise.resolve()方法返回一个新的 Promise 对象,状态为resolved

const p = Promise.resolve('Hello');

p.then(function (s) {
  console.log(s)
});
// Hello

上面代码生成一个新的 Promise 对象的实例p。由于字符串Hello不属于异步操作(判断方法是字符串对象不具有 then 方法),返回 Promise 实例的状态从一生成就是resolved,所以回调函数会立即执行。Promise.resolve()方法的参数,会同时传给回调函数。

(4)不带有任何参数

Promise.resolve()方法允许调用时不带参数,直接返回一个resolved状态的 Promise 对象。

所以,如果希望得到一个 Promise 对象,比较方便的方法就是直接调用Promise.resolve()方法。

const p = Promise.resolve();

p.then(function () {
  // ...
});

上面代码的变量p就是一个 Promise 对象。

需要注意的是,立即resolve()Promise 对象,是在本轮“事件循环”(event loop)的结束时执行,而不是在下一轮“事件循环”的开始时。

setTimeout(function () {
  console.log('three');
}, 0);

Promise.resolve().then(function () {
  console.log('two');
});

console.log('one');

// one
// two
// three

上面代码中,setTimeout(fn, 0)在下一轮“事件循环”开始时执行,Promise.resolve()在本轮“事件循环”结束时执行,console.log('one')则是立即执行,因此最先输出。

11. Promise.reject()

Promise.reject(reason)方法也会返回一个新的 Promise 实例,该实例的状态为rejected

const p = Promise.reject('出错了');
// 等同于
const p = new Promise((resolve, reject) => reject('出错了'))

p.then(null, function (s) {
  console.log(s)
});
// 出错了

上面代码生成一个 Promise 对象的实例p,状态为rejected,回调函数会立即执行。

Promise.reject()方法的参数,会原封不动地作为reject的理由,变成后续方法的参数。

Promise.reject('出错了')
.catch(e => {
  console.log(e === '出错了')
})
// true

上面代码中,Promise.reject()方法的参数是一个字符串,后面catch()方法的参数e就是这个字符串。

12. 应用

加载图片

我们可以将图片的加载写成一个Promise,一旦加载完成,Promise的状态就发生变化。

const preloadImage = function (path) {
  return new Promise(function (resolve, reject) {
    const image = new Image();
    image.onload  = resolve;
    image.onerror = reject;
    image.src = path;
  });
};

Generator 函数与 Promise 的结合

使用 Generator 函数管理流程,遇到异步操作的时候,通常返回一个Promise对象。

function getFoo () {
  return new Promise(function (resolve, reject){
    resolve('foo');
  });
}

const g = function* () {
  try {
    const foo = yield getFoo();
    console.log(foo);
  } catch (e) {
    console.log(e);
  }
};

function run (generator) {
  const it = generator();

  function go(result) {
    if (result.done) return result.value;

    return result.value.then(function (value) {
      return go(it.next(value));
    }, function (error) {
      return go(it.throw(error));
    });
  }

  go(it.next());
}

run(g);

上面代码的 Generator 函数 g 之中,有一个异步操作 getFoo,它返回的就是一个 Promise 对象。函数 run 用来处理这个 Promise 对象,并调用下一个 next 方法。

13. Promise.try()

实际开发中,经常遇到一种情况:不知道或者不想区分,函数f是同步函数还是异步操作,但是想用 Promise 来处理它。因为这样就可以不管f是否包含异步操作,都用then方法指定下一步流程,用catch方法处理f抛出的错误。一般就会采用下面的写法。

Promise.resolve().then(f)

上面的写法有一个缺点,就是如果f是同步函数,那么它会在本轮事件循环的末尾执行。

const f = () => console.log('now');
Promise.resolve().then(f);
console.log('next');
// next
// now

上面代码中,函数f是同步的,但是用 Promise 包装了以后,就变成异步执行了。

那么有没有一种方法,让同步函数同步执行,异步函数异步执行,并且让它们具有统一的 API 呢?回答是可以的,并且还有两种写法。第一种写法是用async函数来写。

const f = () => console.log('now');
(async () => f())();
console.log('next');
// now
// next

上面代码中,第二行是一个立即执行的匿名函数,会立即执行里面的 async 函数,因此如果 f 是同步的,就会得到同步的结果;如果 f 是异步的,就可以用 then 指定下一步,就像下面的写法。

(async () => f())()
.then(...)

需要注意的是,async () => f() 会吃掉 f() 抛出的错误。所以,如果想捕获错误,要使用 promise.catch 方法。

(async () => f())()
.then(...)
.catch(...)

第二种写法是使用new Promise()

const f = () => console.log('now');
(
  () => new Promise(
    resolve => resolve(f())
  )
)();
console.log('next');
// now
// next

上面代码也是使用立即执行的匿名函数,执行new Promise()。这种情况下,同步函数也是同步执行的。

鉴于这是一个很常见的需求,所以现在有一个提案,提供Promise.try方法替代上面的写法。

const f = () => console.log('now');
Promise.try(f);
console.log('next');
// now
// next

事实上,Promise.try 存在已久,PromiseBluebirdQwhen,早就提供了这个方法。

由于 Promise.try 为所有操作提供了统一的处理机制,所以如果想用 then 方法管理流程,最好都用 Promise.try 包装一下。这样有许多好处,其中一点就是可以更好地管理异常。

function getUsername(userId) {
  return database.users.get({id: userId})
  .then(function(user) {
    return user.name;
  });
}

上面代码中,database.users.get()返回一个 Promise 对象,如果抛出异步错误,可以用catch方法捕获,就像下面这样写。

database.users.get({id: userId})
.then(...)
.catch(...)

但是database.users.get()可能还会抛出同步错误(比如数据库连接错误,具体要看实现方法),这时你就不得不用try...catch去捕获。

try {
  database.users.get({id: userId})
  .then(...)
  .catch(...)
} catch (e) {
  // ...
}

上面这样的写法就很笨拙了,这时就可以统一用promise.catch()捕获所有同步和异步的错误。

Promise.try(() => database.users.get({id: userId}))
  .then(...)
  .catch(...)

事实上,Promise.try就是模拟try代码块,就像promise.catch模拟的是catch代码块。

async 函数

1. 含义

ES2017 标准引入了 async 函数,使得异步操作变得更加方便。

async 函数是什么?一句话,它就是 Generator 函数的语法糖。

前文有一个 Generator 函数,依次读取两个文件。

const fs = require('fs');

const readFile = function (fileName) {
  return new Promise(function (resolve, reject) {
    fs.readFile(fileName, function(error, data) {
      if (error) return reject(error);
      resolve(data);
    });
  });
};

const gen = function* () {
  const f1 = yield readFile('/etc/fstab');
  const f2 = yield readFile('/etc/shells');
  console.log(f1.toString());
  console.log(f2.toString());
};

上面代码的函数gen可以写成async函数,就是下面这样。

const asyncReadFile = async function () {
  const f1 = await readFile('/etc/fstab');
  const f2 = await readFile('/etc/shells');
  console.log(f1.toString());
  console.log(f2.toString());
};

一比较就会发现,async 函数就是将 Generator 函数的星号(*)替换成 async,将 yield 替换成 await,仅此而已。

async 函数对 Generator 函数的改进,体现在以下四点。

(1)内置执行器。

Generator 函数的执行必须靠执行器,所以才有了 co 模块,而 async 函数自带执行器。也就是说,async 函数的执行,与普通函数一模一样,只要一行。

asyncReadFile();

上面的代码调用了asyncReadFile函数,然后它就会自动执行,输出最后结果。这完全不像 Generator 函数,需要调用next方法,或者用co模块,才能真正执行,得到最后结果。

(2)更好的语义。

asyncawait,比起星号和 yield,语义更清楚了。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。

(3)更广的适用性。

co 模块约定,yield 命令后面只能是 Thunk 函数或 Promise 对象,而 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,但这时会自动转成立即 resolved 的 Promise 对象)。

(4)返回值是 Promise

async 函数的返回值是 Promise 对象,这比 Generator 函数的返回值是 Iterator 对象方便多了。你可以用 then 方法指定下一步的操作。

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

2. 基本用法

async 函数返回一个 Promise 对象,可以使用 then 方法添加回调函数。当函数执行的时候,一旦遇到 await 就会先返回,等到异步操作完成,再接着执行函数体内后面的语句。

下面是一个例子。

async function getStockPriceByName(name) {
  const symbol = await getStockSymbol(name);
  const stockPrice = await getStockPrice(symbol);
  return stockPrice;
}

getStockPriceByName('goog').then(function (result) {
  console.log(result);
});

上面代码是一个获取股票报价的函数,函数前面的async关键字,表明该函数内部有异步操作。调用该函数时,会立即返回一个Promise对象。

下面是另一个例子,指定多少毫秒后输出一个值。

function timeout(ms) {
  return new Promise((resolve) => {
    setTimeout(resolve, ms);
  });
}

async function asyncPrint(value, ms) {
  await timeout(ms);
  console.log(value);
}

asyncPrint('hello world', 50);

上面代码指定 50 毫秒以后,输出hello world

由于async函数返回的是 Promise 对象,可以作为await命令的参数。所以,上面的例子也可以写成下面的形式。

async function timeout(ms) {
  await new Promise((resolve) => {
    setTimeout(resolve, ms);
  });
}

async function asyncPrint(value, ms) {
  await timeout(ms);
  console.log(value);
}

asyncPrint('hello world', 50);

async 函数有多种使用形式。

// 函数声明
async function foo() {}

// 函数表达式
const foo = async function () {};

// 对象的方法
let obj = { async foo() {} };
obj.foo().then(...)

// Class 的方法
class Storage {
  constructor() {
    this.cachePromise = caches.open('avatars');
  }

  async getAvatar(name) {
    const cache = await this.cachePromise;
    return cache.match(`/avatars/${name}.jpg`);
  }
}

const storage = new Storage();
storage.getAvatar('jake').then();

// 箭头函数
const foo = async () => {};

3. 语法

async函数的语法规则总体上比较简单,难点是错误处理机制。

返回 Promise 对象

async函数返回一个 Promise 对象。

async函数内部return语句返回的值,会成为then方法回调函数的参数。

async function f() {
  return 'hello world';
}

f().then(v => console.log(v))
// "hello world"

上面代码中,函数f内部return命令返回的值,会被then方法回调函数接收到。

async函数内部抛出错误,会导致返回的 Promise 对象变为reject状态。抛出的错误对象会被catch方法回调函数接收到。

async function f() {
  throw new Error('出错了');
}

f().then(
  v => console.log('resolve', v),
  e => console.log('reject', e)
)
//reject Error: 出错了

Promise 对象的状态变化

async函数返回的 Promise 对象,必须等到内部所有await命令后面的 Promise 对象执行完,才会发生状态改变,除非遇到return语句或者抛出错误。也就是说,只有async函数内部的异步操作执行完,才会执行then方法指定的回调函数。

下面是一个例子。

async function getTitle(url) {
  let response = await fetch(url);
  let html = await response.text();
  return html.match(/([\s\S]+)<\/title></span><span class="token regex-delimiter">/</span><span class="token regex-flags">i</span></span><span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">getTitle</span><span class="token punctuation">(</span><span class="token string">'https://tc39.github.io/ecma262/'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span>console<span class="token punctuation">.</span>log<span class="token punctuation">)</span>
<span class="token comment">// "ECMAScript 2017 Language Specification"</span>
</code></pre> 
  <p>上面代码中,函数<code>getTitle</code>内部有三个操作:抓取网页、取出文本、匹配页面标题。只有这三个操作全部完成,才会执行<code>then</code>方法里面的<code>console.log</code>。</p> 
  <p><strong>await 命令</strong></p> 
  <p>正常情况下,<code>await</code>命令后面是一个 <code>Promise</code> 对象,返回该对象的结果。如果不是 <code>Promise</code> 对象,就直接返回对应的值。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 等同于</span>
  <span class="token comment">// return 123;</span>
  <span class="token keyword">return</span> <span class="token keyword">await</span> <span class="token number">123</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">v</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// 123</span>
</code></pre> 
  <p>上面代码中,<code>await</code>命令的参数是数值<code>123</code>,这时等同于<code>return 123</code>。</p> 
  <p>另一种情况是,<code>await</code>命令后面是一个<code>thenable</code>对象(即定义了<code>then</code>方法的对象),那么<code>await</code>会将其等同于 <code>Promise</code> 对象。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">class</span> <span class="token class-name">Sleep</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">timeout</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>timeout <span class="token operator">=</span> timeout<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> startTime <span class="token operator">=</span> Date<span class="token punctuation">.</span><span class="token function">now</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">setTimeout</span><span class="token punctuation">(</span>
      <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token function">resolve</span><span class="token punctuation">(</span>Date<span class="token punctuation">.</span><span class="token function">now</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span> startTime<span class="token punctuation">)</span><span class="token punctuation">,</span>
      <span class="token keyword">this</span><span class="token punctuation">.</span>timeout
    <span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> sleepTime <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">new</span> <span class="token class-name">Sleep</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>sleepTime<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 1000</span>
</code></pre> 
  <p>上面代码中,<code>await</code>命令后面是一个<code>Sleep</code>对象的实例。这个实例不是 <code>Promise</code> 对象,但是因为定义了<code>then</code>方法,<code>await</code>会将其视为 <code>Promise</code> 处理。</p> 
  <p>这个例子还演示了如何实现休眠效果。JavaScript 一直没有休眠的语法,但是借助<code>await</code>命令就可以让程序停顿指定的时间。下面给出了一个简化的<code>sleep</code>实现。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">function</span> <span class="token function">sleep</span><span class="token punctuation">(</span><span class="token parameter">interval</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token parameter">resolve</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
    <span class="token function">setTimeout</span><span class="token punctuation">(</span>resolve<span class="token punctuation">,</span> interval<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token comment">// 用法</span>
<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">one2FiveInAsync</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator"><=</span> <span class="token number">5</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">await</span> <span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token function">one2FiveInAsync</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p><code>await</code>命令后面的 <code>Promise</code> 对象如果变为<code>reject</code>状态,则<code>reject</code>的参数会被<code>catch</code>方法的回调函数接收到。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">reject</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">v</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">e</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// 出错了</span>
</code></pre> 
  <p>注意,上面代码中,<code>await</code>语句前面没有<code>return</code>,但是<code>reject</code>方法的参数依然传入了<code>catch</code>方法的回调函数。这里如果在<code>await</code>前面加上<code>return</code>,效果是一样的。</p> 
  <p>任何一个<code>await</code>语句后面的 <code>Promise</code> 对象变为<code>reject</code>状态,那么整个<code>async</code>函数都会中断执行。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">reject</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token string">'hello world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 不会执行</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>上面代码中,第二个<code>await</code>语句是不会执行的,因为第一个<code>await</code>语句状态变成了<code>reject</code>。</p> 
  <p>有时,我们希望即使前一个异步操作失败,也不要中断后面的异步操作。这时可以将第一个<code>await</code>放在<code>try...catch</code>结构里面,这样不管这个异步操作是否成功,第二个<code>await</code>都会执行。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">reject</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token string">'hello world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">v</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// hello world</span>
</code></pre> 
  <p>另一种方法是<code>await</code>后面的 <code>Promise</code> 对象再跟一个<code>catch</code>方法,处理前面可能出现的错误。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">reject</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span>
    <span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">e</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token string">'hello world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">v</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// 出错了</span>
<span class="token comment">// hello world</span>
</code></pre> 
  <p><strong>错误处理</strong></p> 
  <p>如果<code>await</code>后面的异步操作出错,那么等同于<code>async</code>函数返回的 <code>Promise</code> 对象被<code>reject</code>。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">await</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">v</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">e</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token comment">// Error:出错了</span>
</code></pre> 
  <p>上面代码中,<code>async</code>函数<code>f</code>执行后,<code>await</code>后面的 <code>Promise</code> 对象会抛出一个错误对象,导致<code>catch</code>方法的回调函数被调用,它的参数就是抛出的错误对象。具体的执行机制,可以参考后文的“async 函数的实现原理”。</p> 
  <p>防止出错的方法,也是将其放在<code>try...catch</code>代码块之中。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> <span class="token keyword">await</span><span class="token punctuation">(</span><span class="token string">'hello world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>如果有多个<code>await</code>命令,可以统一放在<code>try...catch</code>结构中。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> val1 <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">firstStep</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">const</span> val2 <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">secondStep</span><span class="token punctuation">(</span>val1<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">const</span> val3 <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">thirdStep</span><span class="token punctuation">(</span>val1<span class="token punctuation">,</span> val2<span class="token punctuation">)</span><span class="token punctuation">;</span>

    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Final: '</span><span class="token punctuation">,</span> val3<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">catch</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">error</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>下面的例子使用<code>try...catch</code>结构,实现多次重复尝试。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">const</span> superagent <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'superagent'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token constant">NUM_RETRIES</span> <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>

<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> i<span class="token punctuation">;</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span>i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> <span class="token constant">NUM_RETRIES</span><span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">try</span> <span class="token punctuation">{</span>
      <span class="token keyword">await</span> superagent<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token string">'http://google.com/this-throws-an-error'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
<span class="token punctuation">}</span>

<span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,如果<code>await</code>操作成功,就会使用<code>break</code>语句退出循环;如果失败,会被<code>catch</code>语句捕捉,然后进入下一轮循环。</p> 
  <p><strong>使用注意点</strong></p> 
  <p>第一点,前面已经说过,<code>await</code>命令后面的<code>Promise</code>对象,运行结果可能是<code>rejected</code>,所以最好把<code>await</code>命令放在<code>try...catch</code>代码块中。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">myFunction</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> <span class="token function">somethingThatReturnsAPromise</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token comment">// 另一种写法</span>

<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">myFunction</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">await</span> <span class="token function">somethingThatReturnsAPromise</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">err</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>第二点,多个<code>await</code>命令后面的异步操作,如果不存在继发关系,最好让它们同时触发。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">let</span> foo <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">getFoo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> bar <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">getBar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,<code>getFoo</code> 和 <code>getBar</code> 是两个独立的异步操作(即互不依赖),被写成继发关系。这样比较耗时,因为只有 <code>getFoo</code> 完成以后,才会执行 <code>getBar</code>,完全可以让它们同时触发。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// 写法一</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">,</span> bar<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">all</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token function">getFoo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token function">getBar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 写法二</span>
<span class="token keyword">let</span> fooPromise <span class="token operator">=</span> <span class="token function">getFoo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> barPromise <span class="token operator">=</span> <span class="token function">getBar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> foo <span class="token operator">=</span> <span class="token keyword">await</span> fooPromise<span class="token punctuation">;</span>
<span class="token keyword">let</span> bar <span class="token operator">=</span> <span class="token keyword">await</span> barPromise<span class="token punctuation">;</span>
</code></pre> 
  <p>上面两种写法,<code>getFoo</code> 和 <code>getBar</code> 都是同时触发,这样就会缩短程序的执行时间。</p> 
  <p>第三点,<code>await</code>命令只能用在<code>async</code>函数之中,如果用在普通函数,就会报错。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">dbFuc</span><span class="token punctuation">(</span><span class="token parameter">db</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> docs <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

  <span class="token comment">// 报错</span>
  docs<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">doc</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> db<span class="token punctuation">.</span><span class="token function">post</span><span class="token punctuation">(</span>doc<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>上面代码会报错,因为<code>await</code>用在普通函数之中了。但是,如果将<code>forEach</code>方法的参数改成<code>async</code>函数,也有问题。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">function</span> <span class="token function">dbFuc</span><span class="token punctuation">(</span><span class="token parameter">db</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">//这里不需要 async</span>
  <span class="token keyword">let</span> docs <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

  <span class="token comment">// 可能得到错误结果</span>
  docs<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">doc</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> db<span class="token punctuation">.</span><span class="token function">post</span><span class="token punctuation">(</span>doc<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>上面代码可能不会正常工作,原因是这时三个<code>db.post()</code>操作将是并发执行,也就是同时执行,而不是继发执行。正确的写法是采用<code>for</code>循环。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">dbFuc</span><span class="token punctuation">(</span><span class="token parameter">db</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> docs <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> doc <span class="token keyword">of</span> docs<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> db<span class="token punctuation">.</span><span class="token function">post</span><span class="token punctuation">(</span>doc<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>另一种方法是使用数组的<code>reduce()</code>方法。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">dbFuc</span><span class="token punctuation">(</span><span class="token parameter">db</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> docs <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

  <span class="token keyword">await</span> docs<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token punctuation">(</span><span class="token parameter">_<span class="token punctuation">,</span> doc</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> _<span class="token punctuation">;</span>
    <span class="token keyword">await</span> db<span class="token punctuation">.</span><span class="token function">post</span><span class="token punctuation">(</span>doc<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>上面例子中,<code>reduce()</code>方法的第一个参数是<code>async</code>函数,导致该函数的第一个参数是前一步操作返回的 <code>Promise</code> 对象,所以必须使用<code>await</code>等待它操作结束。另外,<code>reduce()</code>方法返回的是<code>docs</code>数组最后一个成员的<code>async</code>函数的执行结果,也是一个 <code>Promise</code> 对象,导致在它前面也必须加上<code>await</code>。</p> 
  <p>上面的<code>reduce()</code>的参数函数里面没有<code>return</code>语句,原因是这个函数的主要目的是<code>db.post()</code>操作,不是返回值。而且<code>async</code>函数不管有没有<code>return</code>语句,总是返回一个 <code>Promise</code> 对象,所以这里的<code>return</code>是不必要的。</p> 
  <p>如果确实希望多个请求并发执行,可以使用<code>Promise.all</code>方法。当三个请求都会<code>resolved</code>时,下面两种写法效果相同。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">dbFuc</span><span class="token punctuation">(</span><span class="token parameter">db</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> docs <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token keyword">let</span> promises <span class="token operator">=</span> docs<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">doc</span><span class="token punctuation">)</span> <span class="token operator">=></span> db<span class="token punctuation">.</span><span class="token function">post</span><span class="token punctuation">(</span>doc<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">let</span> results <span class="token operator">=</span> <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">all</span><span class="token punctuation">(</span>promises<span class="token punctuation">)</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>results<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 或者使用下面的写法</span>

<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">dbFuc</span><span class="token punctuation">(</span><span class="token parameter">db</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> docs <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token keyword">let</span> promises <span class="token operator">=</span> docs<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">doc</span><span class="token punctuation">)</span> <span class="token operator">=></span> db<span class="token punctuation">.</span><span class="token function">post</span><span class="token punctuation">(</span>doc<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">let</span> results <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> promise <span class="token keyword">of</span> promises<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    results<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token keyword">await</span> promise<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>results<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>第四点,<code>async</code> 函数可以保留运行堆栈。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">const</span> <span class="token function-variable function">a</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
  <span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token function">c</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,函数<code>a</code>内部运行了一个异步任务<code>b()</code>。当<code>b()</code>运行的时候,函数<code>a()</code>不会中断,而是继续执行。等到<code>b()</code>运行结束,可能<code>a()</code>早就运行结束了,<code>b()</code>所在的上下文环境已经消失了。如果<code>b()</code>或<code>c()</code>报错,错误堆栈将不包括<code>a()</code>。</p> 
  <p>现在将这个例子改成<code>async</code>函数。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">const</span> <span class="token function-variable function">a</span> <span class="token operator">=</span> <span class="token keyword">async</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
  <span class="token keyword">await</span> <span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token function">c</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,<code>b()</code>运行的时候,<code>a()</code>是暂停执行,上下文环境都保存着。一旦<code>b()</code>或<code>c()</code>报错,错误堆栈将包括<code>a()</code>。</p> 
  <h3><strong>4. async 函数的实现原理</strong></h3> 
  <p><code>async</code> 函数的实现原理,就是将 <code>Generator</code> 函数和自动执行器,包装在一个函数里。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token parameter">args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>

<span class="token comment">// 等同于</span>

<span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token parameter">args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token function">spawn</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token operator">*</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// ...</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>所有的<code>async</code>函数都可以写成上面的第二种形式,其中的<code>spawn</code>函数就是自动执行器。</p> 
  <p>下面给出<code>spawn</code>函数的实现,基本就是前文自动执行器的翻版。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">function</span> <span class="token function">spawn</span><span class="token punctuation">(</span><span class="token parameter">genF</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> gen <span class="token operator">=</span> <span class="token function">genF</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">function</span> <span class="token function">step</span><span class="token punctuation">(</span><span class="token parameter">nextF</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">let</span> next<span class="token punctuation">;</span>
      <span class="token keyword">try</span> <span class="token punctuation">{</span>
        next <span class="token operator">=</span> <span class="token function">nextF</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token function">reject</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
      <span class="token keyword">if</span><span class="token punctuation">(</span>next<span class="token punctuation">.</span>done<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token function">resolve</span><span class="token punctuation">(</span>next<span class="token punctuation">.</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
      Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span>next<span class="token punctuation">.</span>value<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">v</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">step</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span>v<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">e</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">step</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> gen<span class="token punctuation">.</span><span class="token function">throw</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token function">step</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> gen<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <h3>5. 与其他异步处理方法的比较</h3> 
  <p>我们通过一个例子,来看 <code>async</code> 函数与 <code>Promise、Generator</code> 函数的比较。</p> 
  <p>假定某个 DOM 元素上面,部署了一系列的动画,前一个动画结束,才能开始后一个。如果当中有一个动画出错,就不再往下执行,返回上一个成功执行的动画的返回值。</p> 
  <p>首先是 <code>Promise</code> 的写法。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">function</span> <span class="token function">chainAnimationsPromise</span><span class="token punctuation">(</span><span class="token parameter">elem<span class="token punctuation">,</span> animations</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

  <span class="token comment">// 变量ret用来保存上一个动画的返回值</span>
  <span class="token keyword">let</span> ret <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>

  <span class="token comment">// 新建一个空的Promise</span>
  <span class="token keyword">let</span> p <span class="token operator">=</span> Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token comment">// 使用then方法,添加所有动画</span>
  <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> anim <span class="token keyword">of</span> animations<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    p <span class="token operator">=</span> p<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">val</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      ret <span class="token operator">=</span> val<span class="token punctuation">;</span>
      <span class="token keyword">return</span> <span class="token function">anim</span><span class="token punctuation">(</span>elem<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token comment">// 返回一个部署了错误捕捉机制的Promise</span>
  <span class="token keyword">return</span> p<span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">e</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">/* 忽略错误,继续执行 */</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> ret<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">}</span>
</code></pre> 
  <p>虽然 <code>Promise</code> 的写法比回调函数的写法大大改进,但是一眼看上去,代码完全都是 <code>Promise</code> 的 API(<code>then</code>、<code>catch</code>等等),操作本身的语义反而不容易看出来。</p> 
  <p>接着是 <code>Generator</code> 函数的写法。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">function</span> <span class="token function">chainAnimationsGenerator</span><span class="token punctuation">(</span><span class="token parameter">elem<span class="token punctuation">,</span> animations</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

  <span class="token keyword">return</span> <span class="token function">spawn</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token operator">*</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> ret <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    <span class="token keyword">try</span> <span class="token punctuation">{</span>
      <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> anim <span class="token keyword">of</span> animations<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        ret <span class="token operator">=</span> <span class="token keyword">yield</span> <span class="token function">anim</span><span class="token punctuation">(</span>elem<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">/* 忽略错误,继续执行 */</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> ret<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">}</span>
</code></pre> 
  <p>上面代码使用 <code>Generator</code> 函数遍历了每个动画,语义比 <code>Promise</code> 写法更清晰,用户定义的操作全部都出现在<code>spawn</code>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 <code>Generator</code> 函数,上面代码的<code>spawn</code>函数就是自动执行器,它返回一个 <code>Promise</code> 对象,而且必须保证<code>yield</code>语句后面的表达式,必须返回一个 <code>Promise</code>。</p> 
  <p>最后是 <code>async</code> 函数的写法。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">chainAnimationsAsync</span><span class="token punctuation">(</span><span class="token parameter">elem<span class="token punctuation">,</span> animations</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> ret <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> anim <span class="token keyword">of</span> animations<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      ret <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">anim</span><span class="token punctuation">(</span>elem<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">/* 忽略错误,继续执行 */</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> ret<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>可以看到 Async 函数的实现最简洁,最符合语义,几乎没有语义不相关的代码。它将 Generator 写法中的自动执行器,改在语言层面提供,不暴露给用户,因此代码量最少。如果使用 Generator 写法,自动执行器需要用户自己提供。</p> 
  <h3>6. 实例:按顺序完成异步操作</h3> 
  <p>实际开发中,经常遇到一组异步操作,需要按照顺序完成。比如,依次远程读取一组 URL,然后按照读取的顺序输出结果。</p> 
  <p>Promise 的写法如下。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">function</span> <span class="token function">logInOrder</span><span class="token punctuation">(</span><span class="token parameter">urls</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 远程读取所有URL</span>
  <span class="token keyword">const</span> textPromises <span class="token operator">=</span> urls<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">url</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">fetch</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">response</span> <span class="token operator">=></span> response<span class="token punctuation">.</span><span class="token function">text</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token comment">// 按次序输出</span>
  textPromises<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">chain<span class="token punctuation">,</span> textPromise</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> chain<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> textPromise<span class="token punctuation">)</span>
      <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">text</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>text<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span> Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>上面代码使用<code>fetch</code>方法,同时远程读取一组 URL。每个<code>fetch</code>操作都返回一个 <code>Promise</code> 对象,放入<code>textPromises</code>数组。然后,<code>reduce</code>方法依次处理每个 <code>Promise</code> 对象,然后使用<code>then</code>,将所有 <code>Promise</code> 对象连起来,因此就可以依次输出结果。</p> 
  <p>这种写法不太直观,可读性比较差。下面是 <code>async</code> 函数实现。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">logInOrder</span><span class="token punctuation">(</span><span class="token parameter">urls</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">const</span> url <span class="token keyword">of</span> urls<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> response <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fetch</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">;</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">await</span> response<span class="token punctuation">.</span><span class="token function">text</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>上面代码确实大大简化,问题是所有远程操作都是继发。只有前一个 URL 返回结果,才会去读取下一个 URL,这样做效率很差,非常浪费时间。我们需要的是并发发出远程请求。</p> 
  <pre><code class="prism language-javascript"><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">logInOrder</span><span class="token punctuation">(</span><span class="token parameter">urls</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 并发读取远程URL</span>
  <span class="token keyword">const</span> textPromises <span class="token operator">=</span> urls<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token parameter">url</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> response <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fetch</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> response<span class="token punctuation">.</span><span class="token function">text</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token comment">// 按次序输出</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">const</span> textPromise <span class="token keyword">of</span> textPromises<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">await</span> textPromise<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>上面代码中,虽然<code>map</code>方法的参数是<code>async</code>函数,但它是并发执行的,因为只有<code>async</code>函数内部是继发执行,外部不受影响。后面的<code>for..of</code>循环内部使用了<code>await</code>,因此实现了按顺序输出。</p> 
  <h3><strong>7. 顶层 await</strong></h3> 
  <p>早期的语法规定是,<code>await</code>命令只能出现在 <code>async</code> 函数内部,否则都会报错。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// 报错</span>
<span class="token keyword">const</span> data <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fetch</span><span class="token punctuation">(</span><span class="token string">'https://api.example.com'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,<code>await</code>命令独立使用,没有放在 <code>async</code> 函数里面,就会报错。</p> 
  <p>从 ES2022 开始,允许在模块的顶层独立使用<code>await</code>命令,使得上面那行代码不会报错了。它的主要目的是使用<code>await</code>解决模块异步加载的问题。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// awaiting.js</span>
<span class="token keyword">let</span> output<span class="token punctuation">;</span>
<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> dynamic <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span>someMission<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> data <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fetch</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">;</span>
  output <span class="token operator">=</span> <span class="token function">someProcess</span><span class="token punctuation">(</span>dynamic<span class="token punctuation">.</span>default<span class="token punctuation">,</span> data<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">export</span> <span class="token punctuation">{</span> output <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,模块<code>awaiting.js</code>的输出值<code>output</code>,取决于异步操作。我们把异步操作包装在一个 <code>async</code> 函数里面,然后调用这个函数,只有等里面的异步操作都执行,变量<code>output</code>才会有值,否则就返回<code>undefined</code>。</p> 
  <p>下面是加载这个模块的写法。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// usage.js</span>
<span class="token keyword">import</span> <span class="token punctuation">{</span> output <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">"./awaiting.js"</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> output <span class="token operator">+</span> value <span class="token punctuation">}</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,<code>outputPlusValue()</code>的执行结果,完全取决于执行的时间。如果<code>awaiting.js</code>里面的异步操作没执行完,加载进来的<code>output</code>的值就是<code>undefined</code>。</p> 
  <p>目前的解决方法,就是让原始模块输出一个 <code>Promise</code> 对象,从这个 <code>Promise</code> 对象判断异步操作有没有结束。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// awaiting.js</span>
<span class="token keyword">let</span> output<span class="token punctuation">;</span>
<span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> dynamic <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span>someMission<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> data <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fetch</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">;</span>
  output <span class="token operator">=</span> <span class="token function">someProcess</span><span class="token punctuation">(</span>dynamic<span class="token punctuation">.</span>default<span class="token punctuation">,</span> data<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">export</span> <span class="token punctuation">{</span> output <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,<code>awaiting.js</code>除了输出<code>output</code>,还默认输出一个 <code>Promise</code> 对象(<code>async</code> 函数立即执行后,返回一个 <code>Promise</code> 对象),从这个对象判断异步操作是否结束。</p> 
  <p>下面是加载这个模块的新的写法。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// usage.js</span>
<span class="token keyword">import</span> promise<span class="token punctuation">,</span> <span class="token punctuation">{</span> output <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">"./awaiting.js"</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> output <span class="token operator">+</span> value <span class="token punctuation">}</span>

promise<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,将<code>awaiting.js</code>对象的输出,放在<code>promise.then()</code>里面,这样就能保证异步操作完成以后,才去读取<code>output</code>。</p> 
  <p>这种写法比较麻烦,等于要求模块的使用者遵守一个额外的使用协议,按照特殊的方法使用这个模块。一旦你忘了要用 <code>Promise</code> 加载,只使用正常的加载方法,依赖这个模块的代码就可能出错。而且,如果上面的<code>usage.js</code>又有对外的输出,等于这个依赖链的所有模块都要使用 <code>Promise</code> 加载。</p> 
  <p>顶层的<code>await</code>命令,就是为了解决这个问题。它保证只有异步操作完成,模块才会输出值。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// awaiting.js</span>
<span class="token keyword">const</span> dynamic <span class="token operator">=</span> <span class="token keyword">import</span><span class="token punctuation">(</span>someMission<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> data <span class="token operator">=</span> <span class="token function">fetch</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">export</span> <span class="token keyword">const</span> output <span class="token operator">=</span> <span class="token function">someProcess</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token keyword">await</span> dynamic<span class="token punctuation">)</span><span class="token punctuation">.</span>default<span class="token punctuation">,</span> <span class="token keyword">await</span> data<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码中,两个异步操作在输出的时候,都加上了<code>await</code>命令。只有等到异步操作完成,这个模块才会输出值。</p> 
  <p>加载这个模块的写法如下。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// usage.js</span>
<span class="token keyword">import</span> <span class="token punctuation">{</span> output <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">"./awaiting.js"</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> output <span class="token operator">+</span> value <span class="token punctuation">}</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">outputPlusValue</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码的写法,与普通的模块加载完全一样。也就是说,模块的使用者完全不用关心,依赖模块的内部有没有异步操作,正常加载即可。</p> 
  <p>这时,模块的加载会等待依赖模块(上例是<code>awaiting.js</code>)的异步操作完成,才执行后面的代码,有点像暂停在那里。所以,它总是会得到正确的<code>output</code>,不会因为加载时机的不同,而得到不一样的值。</p> 
  <p>注意,顶层<code>await</code>只能用在 ES6 模块,不能用在 CommonJS 模块。这是因为 CommonJS 模块的<code>require()</code>是同步加载,如果有顶层<code>await</code>,就没法处理加载了。</p> 
  <p>下面是顶层<code>await</code>的一些使用场景。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// import() 方法加载</span>
<span class="token keyword">const</span> strings <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">/i18n/</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>navigator<span class="token punctuation">.</span>language<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 数据库操作</span>
<span class="token keyword">const</span> connection <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">dbConnector</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 依赖回滚</span>
<span class="token keyword">let</span> jQuery<span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
  jQuery <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'https://cdn-a.com/jQuery'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">{</span>
  jQuery <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'https://cdn-b.com/jQuery'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> 
  <p>注意,如果加载多个包含顶层<code>await</code>命令的模块,加载命令是同步执行的。</p> 
  <pre><code class="prism language-javascript"><span class="token comment">// x.js</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">"X1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">await</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token parameter">r</span> <span class="token operator">=></span> <span class="token function">setTimeout</span><span class="token punctuation">(</span>r<span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">"X2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// y.js</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">"Y"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// z.js</span>
<span class="token keyword">import</span> <span class="token string">"./x.js"</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token string">"./y.js"</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">"Z"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> 
  <p>上面代码有三个模块,最后的<code>z.js</code>加载<code>x.js</code>和<code>y.js</code>,打印结果是X1、Y、X2、Z。这说明,<code>z.js</code>并没有等待<code>x.js</code>加载完成,再去加载<code>y.js</code>。</p> 
  <p>顶层的<code>await</code>命令有点像,交出代码的执行权给其他的模块加载,等异步操作完成后,再拿回执行权,继续向下执行。</p> 
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<a class="tag" taget="_blank" href="/search/%E9%9D%A2%E8%AF%95/1.htm">面试</a><a class="tag" taget="_blank" href="/search/%E5%AD%A6%E4%B9%A0%E8%B7%AF%E7%BA%BF/1.htm">学习路线</a><a class="tag" taget="_blank" href="/search/%E9%98%BF%E9%87%8C%E5%B7%B4%E5%B7%B4/1.htm">阿里巴巴</a><a class="tag" taget="_blank" href="/search/vue.js/1.htm">vue.js</a><a class="tag" taget="_blank" href="/search/%E5%89%8D%E7%AB%AF/1.htm">前端</a><a class="tag" taget="_blank" href="/search/javascript/1.htm">javascript</a><a class="tag" taget="_blank" href="/search/intellij-idea/1.htm">intellij-idea</a><a class="tag" taget="_blank" href="/search/%E6%9E%B6%E6%9E%84/1.htm">架构</a>
                        <div>目录结构(根目录开始)├──api#所有请求│└──user.js#用户请求api├──store#全局store管理│└──modules#api模块││└──user.js#用户请求模块│└──index.js#状态管理初始化├──utils#全局公用方法│├──auth.js#本地缓存│├──env.js#公共参数│└──request.js#网络请求中间件└──main.js#入口文件加载</div>
                    </li>
                    <li><a href="/article/1896519816378904576.htm"
                           title="Vue 2 vs Vue 3:深入对比 vue.config.js 配置差异与迁移指南" target="_blank">Vue 2 vs Vue 3:深入对比 vue.config.js 配置差异与迁移指南</a>
                        <span class="text-muted">奶凶的小熊崽</span>
<a class="tag" taget="_blank" href="/search/vue/1.htm">vue</a><a class="tag" taget="_blank" href="/search/javascript/1.htm">javascript</a><a class="tag" taget="_blank" href="/search/vue.js/1.htm">vue.js</a><a class="tag" taget="_blank" href="/search/%E5%89%8D%E7%AB%AF/1.htm">前端</a><a class="tag" taget="_blank" href="/search/javascript/1.htm">javascript</a><a class="tag" taget="_blank" href="/search/webpack/1.htm">webpack</a>
                        <div>目前Vue版本已发布至Vue3.x,而Vue2.x和Vue3.x在项目配置和开发方式上有些差异。在Vue3中,vue.config.js的配置文件结构和Vue2是相似的,以下是一个适用于Vue3的完整配置文件示例,兼顾开发环境、生产环境优化以及常用功能配置。适用于Vue3的vue.config.js配置文件示例:constpath=require('path');constCompressionW</div>
                    </li>
                                <li><a href="/article/117.htm"
                                       title="Algorithm" target="_blank">Algorithm</a>
                                    <span class="text-muted">香水浓</span>
<a class="tag" taget="_blank" href="/search/java/1.htm">java</a><a class="tag" taget="_blank" href="/search/Algorithm/1.htm">Algorithm</a>
                                    <div>冒泡排序 
 

	public static void sort(Integer[] param) {
		for (int i = param.length - 1; i > 0; i--) {
			for (int j = 0; j < i; j++) {
				int current = param[j];
				int next = param[j + 1];</div>
                                </li>
                                <li><a href="/article/244.htm"
                                       title="mongoDB 复杂查询表达式" target="_blank">mongoDB 复杂查询表达式</a>
                                    <span class="text-muted">开窍的石头</span>
<a class="tag" taget="_blank" href="/search/mongodb/1.htm">mongodb</a>
                                    <div>1:count 
   Pg: db.user.find().count(); 
   统计多少条数据 
2:不等于$ne 
   Pg: db.user.find({_id:{$ne:3}},{name:1,sex:1,_id:0}); 
   查询id不等于3的数据。 
3:大于$gt $gte(大于等于) 
&n</div>
                                </li>
                                <li><a href="/article/371.htm"
                                       title="Jboss Java heap space异常解决方法, jboss OutOfMemoryError : PermGen space" target="_blank">Jboss Java heap space异常解决方法, jboss OutOfMemoryError : PermGen space</a>
                                    <span class="text-muted">0624chenhong</span>
<a class="tag" taget="_blank" href="/search/jvm/1.htm">jvm</a><a class="tag" taget="_blank" href="/search/jboss/1.htm">jboss</a>
                                    <div>转自 
http://blog.csdn.net/zou274/article/details/5552630 
 
解决办法: 
 
window->preferences->java->installed jres->edit jre 
把default vm arguments 的参数设为-Xms64m -Xmx512m 
 
 
 
----------------</div>
                                </li>
                                <li><a href="/article/498.htm"
                                       title="文件上传 下载 解析 相对路径" target="_blank">文件上传 下载 解析 相对路径</a>
                                    <span class="text-muted">不懂事的小屁孩</span>
<a class="tag" taget="_blank" href="/search/%E6%96%87%E4%BB%B6%E4%B8%8A%E4%BC%A0/1.htm">文件上传</a>
                                    <div>有点坑吧,弄这么一个简单的东西弄了一天多,身边还有大神指导着,网上各种百度着。 
下面总结一下遇到的问题: 
 
文件上传,在页面上传的时候,不要想着去操作绝对路径,浏览器会对客户端的信息进行保护,避免用户信息收到攻击。 
在上传图片,或者文件时,使用form表单来操作。 
前台通过form表单传输一个流到后台,而不是ajax传递参数到后台,代码如下: 
 

<form action=&</div>
                                </li>
                                <li><a href="/article/625.htm"
                                       title="怎么实现qq空间批量点赞" target="_blank">怎么实现qq空间批量点赞</a>
                                    <span class="text-muted">换个号韩国红果果</span>
<a class="tag" taget="_blank" href="/search/qq/1.htm">qq</a>
                                    <div>纯粹为了好玩!! 
逻辑很简单 
1 打开浏览器console;输入以下代码。 
先上添加赞的代码 
 

var tools={};
		//添加所有赞
		function  init(){
			document.body.scrollTop=10000;
				setTimeout(function(){document.body.scrollTop=0;},2000);//加</div>
                                </li>
                                <li><a href="/article/752.htm"
                                       title="判断是否为中文" target="_blank">判断是否为中文</a>
                                    <span class="text-muted">灵静志远</span>
<a class="tag" taget="_blank" href="/search/%E4%B8%AD%E6%96%87/1.htm">中文</a>
                                    <div>方法一: 
public class Zhidao { 
 public static void main(String args[]) { 
 String s = "sdf灭礌 kjl d{';\fdsjlk是"; 
 int n=0; 
 for(int i=0; i<s.length(); i++) { 
 n = (int)s.charAt(i); 
 if((</div>
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                                <li><a href="/article/879.htm"
                                       title="一个电话面试后总结" target="_blank">一个电话面试后总结</a>
                                    <span class="text-muted">a-john</span>
<a class="tag" taget="_blank" href="/search/%E9%9D%A2%E8%AF%95/1.htm">面试</a>
                                    <div>今天,接了一个电话面试,对于还是初学者的我来说,紧张了半天。 
面试的问题分了层次,对于一类问题,由简到难。自己觉得回答不好的地方作了一下总结: 
  
在谈到集合类的时候,举几个常用的集合类,想都没想,直接说了list,map。 
  
然后对list和map分别举几个类型: 
  
list方面:ArrayList,LinkedList。在谈到他们的区别时,愣住了</div>
                                </li>
                                <li><a href="/article/1006.htm"
                                       title="MSSQL中Escape转义的使用" target="_blank">MSSQL中Escape转义的使用</a>
                                    <span class="text-muted">aijuans</span>
<a class="tag" taget="_blank" href="/search/MSSQL/1.htm">MSSQL</a>
                                    <div>IF OBJECT_ID('tempdb..#ABC') is not null
drop table tempdb..#ABC


create table #ABC
(
PATHNAME NVARCHAR(50)
)


insert into #ABC
SELECT N'/ABCDEFGHI' 
UNION ALL SELECT N'/ABCDGAFGASASSDFA' 
UNION ALL</div>
                                </li>
                                <li><a href="/article/1133.htm"
                                       title="一个简单的存储过程" target="_blank">一个简单的存储过程</a>
                                    <span class="text-muted">asialee</span>
<a class="tag" taget="_blank" href="/search/mysql/1.htm">mysql</a><a class="tag" taget="_blank" href="/search/%E5%AD%98%E5%82%A8%E8%BF%87%E7%A8%8B/1.htm">存储过程</a><a class="tag" taget="_blank" href="/search/%E6%9E%84%E9%80%A0%E6%95%B0%E6%8D%AE/1.htm">构造数据</a><a class="tag" taget="_blank" href="/search/%E6%89%B9%E9%87%8F%E6%8F%92%E5%85%A5/1.htm">批量插入</a>
                                    <div>           今天要批量的生成一批测试数据,其中中间有部分数据是变化的,本来想写个程序来生成的,后来想到存储过程就可以搞定,所以随手写了一个,记录在此: 
  
         
DELIMITER $$  
DROP PROCEDURE IF EXISTS inse</div>
                                </li>
                                <li><a href="/article/1260.htm"
                                       title="annot convert from HomeFragment_1 to Fragment" target="_blank">annot convert from HomeFragment_1 to Fragment</a>
                                    <span class="text-muted">百合不是茶</span>
<a class="tag" taget="_blank" href="/search/android/1.htm">android</a><a class="tag" taget="_blank" href="/search/%E5%AF%BC%E5%8C%85%E9%94%99%E8%AF%AF/1.htm">导包错误</a>
                                    <div>创建了几个类继承Fragment, 需要将创建的类存储在ArrayList<Fragment>中; 出现不能将new 出来的对象放到队列中,原因很简单; 
    创建类时引入包是:import android.app.Fragment; 
  
   创建队列和对象时使用的包是:import android.support.v4.ap</div>
                                </li>
                                <li><a href="/article/1387.htm"
                                       title="Weblogic10两种修改端口的方法" target="_blank">Weblogic10两种修改端口的方法</a>
                                    <span class="text-muted">bijian1013</span>
<a class="tag" taget="_blank" href="/search/weblogic/1.htm">weblogic</a><a class="tag" taget="_blank" href="/search/%E7%AB%AF%E5%8F%A3%E5%8F%B7/1.htm">端口号</a><a class="tag" taget="_blank" href="/search/%E9%85%8D%E7%BD%AE%E7%AE%A1%E7%90%86/1.htm">配置管理</a><a class="tag" taget="_blank" href="/search/config.xml/1.htm">config.xml</a>
                                    <div>一.进入控制台进行修改    1.进入控制台:  http://127.0.0.1:7001/console     2.展开左边树菜单         域结构->环境->服务器-->点击AdminServer(管理) &</div>
                                </li>
                                <li><a href="/article/1514.htm"
                                       title="mysql 操作指令" target="_blank">mysql 操作指令</a>
                                    <span class="text-muted">征客丶</span>
<a class="tag" taget="_blank" href="/search/mysql/1.htm">mysql</a>
                                    <div>一、连接mysql 
进入 mysql 的安装目录; 
$ bin/mysql -p [host IP 如果是登录本地的mysql 可以不写 -p 直接 -u] -u [userName] -p 
输入密码,回车,接连; 
 
二、权限操作[如果你很了解mysql数据库后,你可以直接去修改系统表,然后用 mysql> flush privileges; 指令让权限生效] 
1、赋权 
mys</div>
                                </li>
                                <li><a href="/article/1641.htm"
                                       title="【Hive一】Hive入门" target="_blank">【Hive一】Hive入门</a>
                                    <span class="text-muted">bit1129</span>
<a class="tag" taget="_blank" href="/search/hive/1.htm">hive</a>
                                    <div>Hive安装与配置 
Hive的运行需要依赖于Hadoop,因此需要首先安装Hadoop2.5.2,并且Hive的启动前需要首先启动Hadoop。 
  Hive安装和配置的步骤 
  
1. 从如下地址下载Hive0.14.0 
  
http://mirror.bit.edu.cn/apache/hive/ 
  
 2.解压hive,在系统变</div>
                                </li>
                                <li><a href="/article/1768.htm"
                                       title="ajax 三种提交请求的方法" target="_blank">ajax 三种提交请求的方法</a>
                                    <span class="text-muted">BlueSkator</span>
<a class="tag" taget="_blank" href="/search/Ajax/1.htm">Ajax</a><a class="tag" taget="_blank" href="/search/jqery/1.htm">jqery</a>
                                    <div>1、ajax 提交请求 
$.ajax({
			type:"post",
			url : "${ctx}/front/Hotel/getAllHotelByAjax.do",
			dataType : "json",
			success : function(result) {
				try {
					for(v</div>
                                </li>
                                <li><a href="/article/1895.htm"
                                       title="mongodb开发环境下的搭建入门" target="_blank">mongodb开发环境下的搭建入门</a>
                                    <span class="text-muted">braveCS</span>
<a class="tag" taget="_blank" href="/search/%E8%BF%90%E7%BB%B4/1.htm">运维</a>
                                    <div>  
linux下安装mongodb 
1)官网下载mongodb-linux-x86_64-rhel62-3.0.4.gz 
2)linux 解压   
gzip -d mongodb-linux-x86_64-rhel62-3.0.4.gz; 
mv mongodb-linux-x86_64-rhel62-3.0.4 mongodb-linux-x86_64-rhel62-</div>
                                </li>
                                <li><a href="/article/2022.htm"
                                       title="编程之美-最短摘要的生成" target="_blank">编程之美-最短摘要的生成</a>
                                    <span class="text-muted">bylijinnan</span>
<a class="tag" taget="_blank" href="/search/java/1.htm">java</a><a class="tag" taget="_blank" href="/search/%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84/1.htm">数据结构</a><a class="tag" taget="_blank" href="/search/%E7%AE%97%E6%B3%95/1.htm">算法</a><a class="tag" taget="_blank" href="/search/%E7%BC%96%E7%A8%8B%E4%B9%8B%E7%BE%8E/1.htm">编程之美</a>
                                    <div>

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

public class ShortestAbstract {

	/**
	 * 编程之美 最短摘要的生成
	 * 扫描过程始终保持一个[pBegin,pEnd]的range,初始化确保[pBegin,pEnd]的ran</div>
                                </li>
                                <li><a href="/article/2149.htm"
                                       title="json数据解析及typeof" target="_blank">json数据解析及typeof</a>
                                    <span class="text-muted">chengxuyuancsdn</span>
<a class="tag" taget="_blank" href="/search/js/1.htm">js</a><a class="tag" taget="_blank" href="/search/typeof/1.htm">typeof</a><a class="tag" taget="_blank" href="/search/json%E8%A7%A3%E6%9E%90/1.htm">json解析</a>
                                    <div>   
       // json格式
	var people='{"authors": [{"firstName": "AAA","lastName": "BBB"},'
		                    +' {"firstName": "CCC&</div>
                                </li>
                                <li><a href="/article/2276.htm"
                                       title="流程系统设计的层次和目标" target="_blank">流程系统设计的层次和目标</a>
                                    <span class="text-muted">comsci</span>
<a class="tag" taget="_blank" href="/search/%E8%AE%BE%E8%AE%A1%E6%A8%A1%E5%BC%8F/1.htm">设计模式</a><a class="tag" taget="_blank" href="/search/%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84/1.htm">数据结构</a><a class="tag" taget="_blank" href="/search/sql/1.htm">sql</a><a class="tag" taget="_blank" href="/search/%E6%A1%86%E6%9E%B6/1.htm">框架</a><a class="tag" taget="_blank" href="/search/%E8%84%9A%E6%9C%AC/1.htm">脚本</a>
                                    <div> 
                              流程系统设计的层次和目标 
 
 
 </div>
                                </li>
                                <li><a href="/article/2403.htm"
                                       title="RMAN List和report 命令" target="_blank">RMAN List和report 命令</a>
                                    <span class="text-muted">daizj</span>
<a class="tag" taget="_blank" href="/search/oracle/1.htm">oracle</a><a class="tag" taget="_blank" href="/search/list/1.htm">list</a><a class="tag" taget="_blank" href="/search/report/1.htm">report</a><a class="tag" taget="_blank" href="/search/rman/1.htm">rman</a>
                                    <div>LIST 命令 
使用RMAN LIST 命令显示有关资料档案库中记录的备份集、代理副本和映像副本的 
信息。使用此命令可列出: 
• RMAN 资料档案库中状态不是AVAILABLE 的备份和副本 
• 可用的且可以用于还原操作的数据文件备份和副本 
• 备份集和副本,其中包含指定数据文件列表或指定表空间的备份 
• 包含指定名称或范围的所有归档日志备份的备份集和副本 
• 由标记、完成时间、可</div>
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                                <li><a href="/article/2530.htm"
                                       title="二叉树:红黑树" target="_blank">二叉树:红黑树</a>
                                    <span class="text-muted">dieslrae</span>
<a class="tag" taget="_blank" href="/search/%E4%BA%8C%E5%8F%89%E6%A0%91/1.htm">二叉树</a>
                                    <div>    红黑树是一种自平衡的二叉树,它的查找,插入,删除操作时间复杂度皆为O(logN),不会出现普通二叉搜索树在最差情况时时间复杂度会变为O(N)的问题. 
    红黑树必须遵循红黑规则,规则如下 
    
1、每个节点不是红就是黑。     2、根总是黑的  &</div>
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                                <li><a href="/article/2657.htm"
                                       title="C语言homework3,7个小题目的代码" target="_blank">C语言homework3,7个小题目的代码</a>
                                    <span class="text-muted">dcj3sjt126com</span>
<a class="tag" taget="_blank" href="/search/c/1.htm">c</a>
                                    <div>1、打印100以内的所有奇数。 
# include <stdio.h>

int main(void)
{
	int i;

	for (i=1; i<=100; i++)
	{
		if (i%2 != 0)
			printf("%d ", i);
	}

	return 0;
}
 
 2、从键盘上输入10个整数,</div>
                                </li>
                                <li><a href="/article/2784.htm"
                                       title="自定义按钮, 图片在上, 文字在下, 居中显示" target="_blank">自定义按钮, 图片在上, 文字在下, 居中显示</a>
                                    <span class="text-muted">dcj3sjt126com</span>
<a class="tag" taget="_blank" href="/search/%E8%87%AA%E5%AE%9A%E4%B9%89/1.htm">自定义</a>
                                    <div>#import <UIKit/UIKit.h>

@interface MyButton : UIButton

-(void)setFrame:(CGRect)frame ImageName:(NSString*)imageName Target:(id)target Action:(SEL)action Title:(NSString*)title Font:(CGFloa</div>
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                                <li><a href="/article/2911.htm"
                                       title="MySQL查询语句练习题,测试足够用了" target="_blank">MySQL查询语句练习题,测试足够用了</a>
                                    <span class="text-muted">flyvszhb</span>
<a class="tag" taget="_blank" href="/search/sql/1.htm">sql</a><a class="tag" taget="_blank" href="/search/mysql/1.htm">mysql</a>
                                    <div>http://blog.sina.com.cn/s/blog_767d65530101861c.html 
1.创建student和score表 
CREATE  TABLE  student ( 
id  INT(10)  NOT NULL  UNIQUE  PRIMARY KEY  , 
name  VARCHAR</div>
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                                <li><a href="/article/3038.htm"
                                       title="转:MyBatis Generator 详解" target="_blank">转:MyBatis Generator 详解</a>
                                    <span class="text-muted">happyqing</span>
<a class="tag" taget="_blank" href="/search/mybatis/1.htm">mybatis</a>
                                    <div>  
MyBatis Generator 详解 
http://blog.csdn.net/isea533/article/details/42102297 
  
MyBatis Generator详解 
http://git.oschina.net/free/Mybatis_Utils/blob/master/MybatisGeneator/MybatisGeneator.</div>
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                                <li><a href="/article/3165.htm"
                                       title="让程序员少走弯路的14个忠告" target="_blank">让程序员少走弯路的14个忠告</a>
                                    <span class="text-muted">jingjing0907</span>
<a class="tag" taget="_blank" href="/search/%E5%B7%A5%E4%BD%9C/1.htm">工作</a><a class="tag" taget="_blank" href="/search/%E8%AE%A1%E5%88%92/1.htm">计划</a><a class="tag" taget="_blank" href="/search/%E5%AD%A6%E4%B9%A0/1.htm">学习</a>
                                    <div>  
无论是谁,在刚进入某个领域之时,有再大的雄心壮志也敌不过眼前的迷茫:不知道应该怎么做,不知道应该做什么。下面是一名软件开发人员所学到的经验,希望能对大家有所帮助 
  
1.不要害怕在工作中学习。  
只要有电脑,就可以通过电子阅读器阅读报纸和大多数书籍。如果你只是做好自己的本职工作以及分配的任务,那是学不到很多东西的。如果你盲目地要求更多的工作,也是不可能提升自己的。放</div>
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2.易用性方面差别也比较大。Nginx对管理员的水平要求比较高,参数比较多,不确定性给运营带来隐患。在NetScaler常见的配置如健康检查,HA等,在Nginx上的配置的实现相对复杂。 
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Besides, I care that how to integrate tightly. 
  
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java.lang.OutOfMemoryError: Java heap space:这种是java堆内存不够,一个原因是真不够,另一个原因是程序中有死循环;     如果是java堆内存不够的话,可以通过调整JVM下面的配置来解决:     <jvm-arg>-Xms3062m</jvm-arg>     <jvm-arg>-Xmx</div>
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继承关系 
public static final class Manifest.permission_group extends Object 
java.lang.Object 
android. Manifest.permission_group 常量 
ACCOUNTS 直接通过统计管理器访问管理的统计 
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