'use strict';// ajax函数将返回Promise对象:function ajax(method, url, data) {    var request = new XMLHttpRequest();    return new Promise(function (resolve, reject) {        request.onreadystatechange = function () {            if (request.readyState === 4) {                if (request.status === 200) {                    resolve(request.responseText);                } else {                    reject(request.status);                }            }        };        request.open(method, url);        request.send(data);    });}var log = document.getElementById('test-promise-ajax-result');var p = ajax('GET', '/api/categories');p.then(function (text) { // 如果AJAX成功，获得响应内容    log.innerText = text;}).catch(function (status) { // 如果AJAX失败，获得响应代码    log.innerText = 'ERROR: ' + status;});

实现原理

//例1function getUserId() {    return new Promise(function(resolve) {        //异步请求        http.get(url, function(results) {            resolve(results.id)        })    })}getUserId().then(function(id) {    //一些处理})function Promise(fn) {    var value = null,        callbacks = [];  //callbacks为数组，因为可能同时有很多个回调    this.then = function (onFulfilled) {        callbacks.push(onFulfilled);    };    function resolve(value) {        callbacks.forEach(function (callback) {            callback(value);        });    }    fn(resolve);}

Promise.all()

var p1 = new Promise(function (resolve, reject) {    setTimeout(resolve, 500, 'P1');});var p2 = new Promise(function (resolve, reject) {    setTimeout(resolve, 600, 'P2');});// 同时执行p1和p2，并在它们都完成后执行then:Promise.all([p1, p2]).then(function (results) {    console.log(results); // 获得一个Array: ['P1', 'P2']});

Promise.race()

var p1 = new Promise(function (resolve, reject) {    setTimeout(resolve, 500, 'P1');});var p2 = new Promise(function (resolve, reject) {    setTimeout(resolve, 600, 'P2');});Promise.race([p1, p2]).then(function (result) {    console.log(result); // 'P1'});//由于p1执行较快，Promise的then()将获得结果'P1'。p2仍在继续执行，但执行结果将被丢弃