ECMAScript 6 学习系列课程 （ES6 Promise对象的使用）

Promise是异步编程的一种解决方案， ES6 这一特性很大程度上提高了开发者的效率，如果用过Angular的同学，一定会用到$q的resolve和reject函数，同理promise也提供原生的解决方案，不需要任何框架支持。

我们下面看一下代码：

function msgAfterTimeout (msg, who, timeout) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(`${msg} Hello ${who}!`), timeout)
    })
}
msgAfterTimeout("", "Foo", 100).then((msg) =>
    msgAfterTimeout(msg, "Bar", 200)
).then((msg) => {
    console.log(`done after 300ms:${msg}`)
})

上面代码如果用ES5实现该如何改写呢？

function msgAfterTimeout (msg, who, timeout, onDone) {
    setTimeout(function () {
        onDone(msg + " Hello " + who + "!");
    }, timeout);
}
msgAfterTimeout("", "Foo", 100, function (msg) {
    msgAfterTimeout(msg, "Bar", 200, function (msg) {
        console.log("done after 300ms:" + msg);
    });
});

对于处理多个异步，类似于$q.all（）的语法在ES6中也是支持的， 例如：

function fetchAsync (url, timeout, onData, onError) {
    …
}
let fetchPromised = (url, timeout) => {
    return new Promise((resolve, reject) => {
        fetchAsync(url, timeout, resolve, reject)
    })
}
Promise.all([
    fetchPromised("http://backend/foo.txt", 500),
    fetchPromised("http://backend/bar.txt", 500),
    fetchPromised("http://backend/baz.txt", 500)
]).then((data) => {
    let [ foo, bar, baz ] = data
    console.log(`success: foo=${foo} bar=${bar} baz=${baz}`)
}, (err) => {
    console.log(`error: ${err}`)
})

对于ES5标准的改写上面代码，是这样的：

function fetchAsync (url, timeout, onData, onError) {
    …
}
function fetchAll (request, onData, onError) {
    var result = [], results = 0;
    for (var i = 0; i < request.length; i++) {
        result[i] = null;
        (function (i) {
            fetchAsync(request[i].url, request[i].timeout, function (data) {
                result[i] = data;
                if (++results === request.length)
                    onData(result);
            }, onError);
        })(i);
    }
}
fetchAll([
    { url: "http://backend/foo.txt", timeout: 500 },
    { url: "http://backend/bar.txt", timeout: 500 },
    { url: "http://backend/baz.txt", timeout: 500 }
], function (data) {
    var foo = data[0], bar = data[1], baz = data[2];
    console.log("success: foo=" + foo + " bar=" + bar + " baz=" + baz);
}, function (err) {
    console.log("error: " + err);
});

是不是很简洁很多呢，尝试一下吧！