'use strict'; // ajax函數將返回Promise對象: 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，獲得響應內容log.innerText = text; }).catch(function (status) { // 如果AJAX失敗，獲得響應代碼log.innerText = 'ERROR: ' + status; }); 實現原理 //例1 function getUserId() {return new Promise(function(resolve) {//異步請求http.get(url, function(results) {resolve(results.id)})}) }getUserId().then(function(id) {//一些處理 })function Promise(fn) {var value = null,callbacks = []; //callbacks為數組，因為可能同時有很多個回調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'); }); // 同時執行p1和p2，并在它們都完成后執行then: Promise.all([p1, p2]).then(function (results) {console.log(results); // 獲得一個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執行較快，Promise的then()將獲得結果'P1'。p2仍在繼續執行，但執行結果將被丟棄

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