1.什么是Promise?

Promise是JS異步編程中的重要概念，異步抽象處理對(duì)象，是目前比較流行Javascript異步編程解決方案之一

2.對(duì)于幾種常見異步編程方案

• 回調(diào)函數(shù)
• 事件監(jiān)聽
• 發(fā)布/訂閱
• Promise對(duì)象

這里就拿回調(diào)函數(shù)說說

1.對(duì)于回調(diào)函數(shù) 我們用Jquery的ajax獲取數(shù)據(jù)時(shí) 都是以回調(diào)函數(shù)方式獲取的數(shù)據(jù)

$.get(url, (data) => {console.log(data) ) 復(fù)制代碼

2.如果說 當(dāng)我們需要發(fā)送多個(gè)異步請(qǐng)求 并且每個(gè)請(qǐng)求之間需要相互依賴 那這時(shí) 我們只能 以嵌套方式來解決 形成 "回調(diào)地獄"

$.get(url, data1 => {console.log(data1)$.get(data1.url, data2 => {console.log(data1)}) }) 復(fù)制代碼

這樣一來，在處理越多的異步邏輯時(shí)，就需要越深的回調(diào)嵌套，這種編碼模式的問題主要有以下幾個(gè)：

• 代碼邏輯書寫順序與執(zhí)行順序不一致，不利于閱讀與維護(hù)。
• 異步操作的順序變更時(shí)，需要大規(guī)模的代碼重構(gòu)。
• 回調(diào)函數(shù)基本都是匿名函數(shù)，bug 追蹤困難。
• 回調(diào)函數(shù)是被第三方庫(kù)代碼（如上例中的 ajax ）而非自己的業(yè)務(wù)代碼所調(diào)用的，造成了 IoC 控制反轉(zhuǎn)。

Promise 處理多個(gè)相互關(guān)聯(lián)的異步請(qǐng)求

1.而我們Promise 可以更直觀的方式 來解決 "回調(diào)地獄"

const request = url => { return new Promise((resolve, reject) => {$.get(url, data => {resolve(data)});}) };// 請(qǐng)求data1 request(url).then(data1 => {return request(data1.url); }).then(data2 => {return request(data2.url); }).then(data3 => {console.log(data3); }).catch(err => throw new Error(err)); 復(fù)制代碼

2.相信大家在 vue/react 都是用axios fetch 請(qǐng)求數(shù)據(jù) 也都支持 Promise API

import axios from 'axios'; axios.get(url).then(data => {console.log(data) }) 復(fù)制代碼

Axios 是一個(gè)基于 promise 的 HTTP 庫(kù)，可以用在瀏覽器和 node.js 中。

3.Promise使用

1.Promise 是一個(gè)構(gòu)造函數(shù)， new Promise 返回一個(gè) promise對(duì)象 接收一個(gè)excutor執(zhí)行函數(shù)作為參數(shù), excutor有兩個(gè)函數(shù)類型形參resolve reject

const promise = new Promise((resolve, reject) => {// 異步處理// 處理結(jié)束后、調(diào)用resolve 或 reject });復(fù)制代碼

2.promise相當(dāng)于一個(gè)狀態(tài)機(jī)

promise的三種狀態(tài)

• pending
• fulfilled
• rejected

1.promise 對(duì)象初始化狀態(tài)為 pending 2.當(dāng)調(diào)用resolve(成功)，會(huì)由pending => fulfilled 3.當(dāng)調(diào)用reject(失敗)，會(huì)由pending => rejected

注意promsie狀態(tài) 只能由 pending => fulfilled/rejected, 一旦修改就不能再變

3.promise對(duì)象方法

1.then方法注冊(cè) 當(dāng)resolve(成功)/reject(失敗)的回調(diào)函數(shù)

// onFulfilled 是用來接收promise成功的值 // onRejected 是用來接收promise失敗的原因 promise.then(onFulfilled, onRejected); 復(fù)制代碼

then方法是異步執(zhí)行的

2.resolve(成功) onFulfilled會(huì)被調(diào)用

const promise = new Promise((resolve, reject) => {resolve('fulfilled'); // 狀態(tài)由 pending => fulfilled }); promise.then(result => { // onFulfilledconsole.log(result); // 'fulfilled' }, reason => { // onRejected 不會(huì)被調(diào)用}) 復(fù)制代碼

3.reject(失敗) onRejected會(huì)被調(diào)用

const promise = new Promise((resolve, reject) => {reject('rejected'); // 狀態(tài)由 pending => rejected }); promise.then(result => { // onFulfilled 不會(huì)被調(diào)用}, reason => { // onRejected console.log(reason); // 'rejected' }) 復(fù)制代碼

4.promise.catch

在鏈?zhǔn)綄懛ㄖ锌梢圆东@前面then中發(fā)送的異常,

promise.catch(onRejected) 相當(dāng)于 promise.then(null, onRrejected);// 注意 // onRejected 不能捕獲當(dāng)前onFulfilled中的異常 promise.then(onFulfilled, onRrejected); // 可以寫成： promise.then(onFulfilled).catch(onRrejected); 復(fù)制代碼

4.promise chain

promise.then方法每次調(diào)用 都返回一個(gè)新的promise對(duì)象 所以可以鏈?zhǔn)綄懛?/p> function taskA() {console.log("Task A"); } function taskB() {console.log("Task B"); } function onRejected(error) {console.log("Catch Error: A or B", error); }var promise = Promise.resolve(); promise.then(taskA).then(taskB).catch(onRejected) // 捕獲前面then方法中的異常 復(fù)制代碼

5.Promise的靜態(tài)方法

1.Promise.resolve 返回一個(gè)fulfilled狀態(tài)的promise對(duì)象

Promise.resolve('hello').then(function(value){console.log(value); });Promise.resolve('hello'); // 相當(dāng)于 const promise = new Promise(resolve => {resolve('hello'); }); 復(fù)制代碼

2.Promise.reject 返回一個(gè)rejected狀態(tài)的promise對(duì)象

Promise.reject(24); new Promise((resolve, reject) => {reject(24); }); 復(fù)制代碼

3.Promise.all 接收一個(gè)promise對(duì)象數(shù)組為參數(shù)

只有全部為resolve才會(huì)調(diào)用 通常會(huì)用來處理 多個(gè)并行異步操作

const p1 = new Promise((resolve, reject) => {resolve(1); });const p2 = new Promise((resolve, reject) => {resolve(2); });const p3 = new Promise((resolve, reject) => {reject(3); });Promise.all([p1, p2, p3]).then(data => { console.log(data); // [1, 2, 3] 結(jié)果順序和promise實(shí)例數(shù)組順序是一致的 }, err => {console.log(err); }); 復(fù)制代碼

4.Promise.race 接收一個(gè)promise對(duì)象數(shù)組為參數(shù)

Promise.race 只要有一個(gè)promise對(duì)象進(jìn)入 FulFilled 或者 Rejected 狀態(tài)的話，就會(huì)繼續(xù)進(jìn)行后面的處理。

function timerPromisefy(delay) {return new Promise(function (resolve, reject) {setTimeout(function () {resolve(delay);}, delay);}); } var startDate = Date.now();Promise.race([timerPromisefy(10),timerPromisefy(20),timerPromisefy(30) ]).then(function (values) {console.log(values); // 10 }); 復(fù)制代碼

5.Promise的finally

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 })); }; 復(fù)制代碼

4. Promise 代碼實(shí)現(xiàn)

/*** Promise 實(shí)現(xiàn) 遵循promise/A+規(guī)范* Promise/A+規(guī)范譯文:* https://malcolmyu.github.io/2015/06/12/Promises-A-Plus/#note-4*/// promise 三個(gè)狀態(tài) const PENDING = "pending"; const FULFILLED = "fulfilled"; const REJECTED = "rejected";function Promise(excutor) {let that = this; // 緩存當(dāng)前promise實(shí)例對(duì)象that.status = PENDING; // 初始狀態(tài)that.value = undefined; // fulfilled狀態(tài)時(shí) 返回的信息that.reason = undefined; // rejected狀態(tài)時(shí) 拒絕的原因that.onFulfilledCallbacks = []; // 存儲(chǔ)fulfilled狀態(tài)對(duì)應(yīng)的onFulfilled函數(shù)that.onRejectedCallbacks = []; // 存儲(chǔ)rejected狀態(tài)對(duì)應(yīng)的onRejected函數(shù)function resolve(value) { // value成功態(tài)時(shí)接收的終值if(value instanceof Promise) {return value.then(resolve, reject);}// 為什么resolve 加setTimeout?// 2.2.4規(guī)范 onFulfilled 和 onRejected 只允許在 execution context 棧僅包含平臺(tái)代碼時(shí)運(yùn)行.// 注1 這里的平臺(tái)代碼指的是引擎、環(huán)境以及 promise 的實(shí)施代碼。實(shí)踐中要確保 onFulfilled 和 onRejected 方法異步執(zhí)行，且應(yīng)該在 then 方法被調(diào)用的那一輪事件循環(huán)之后的新執(zhí)行棧中執(zhí)行。setTimeout(() => {// 調(diào)用resolve 回調(diào)對(duì)應(yīng)onFulfilled函數(shù)if (that.status === PENDING) {// 只能由pending狀態(tài) => fulfilled狀態(tài) (避免調(diào)用多次resolve reject)that.status = FULFILLED;that.value = value;that.onFulfilledCallbacks.forEach(cb => cb(that.value));}});}function reject(reason) { // reason失敗態(tài)時(shí)接收的拒因setTimeout(() => {// 調(diào)用reject 回調(diào)對(duì)應(yīng)onRejected函數(shù)if (that.status === PENDING) {// 只能由pending狀態(tài) => rejected狀態(tài) (避免調(diào)用多次resolve reject)that.status = REJECTED;that.reason = reason;that.onRejectedCallbacks.forEach(cb => cb(that.reason));}});}// 捕獲在excutor執(zhí)行器中拋出的異常// new Promise((resolve, reject) => {// throw new Error('error in excutor')// })try {excutor(resolve, reject);} catch (e) {reject(e);} }/*** resolve中的值幾種情況：* 1.普通值* 2.promise對(duì)象* 3.thenable對(duì)象/函數(shù)*//*** 對(duì)resolve 進(jìn)行改造增強(qiáng) 針對(duì)resolve中不同值情況 進(jìn)行處理* @param {promise} promise2 promise1.then方法返回的新的promise對(duì)象* @param {[type]} x promise1中onFulfilled的返回值* @param {[type]} resolve promise2的resolve方法* @param {[type]} reject promise2的reject方法*/ function resolvePromise(promise2, x, resolve, reject) {if (promise2 === x) { // 如果從onFulfilled中返回的x 就是promise2 就會(huì)導(dǎo)致循環(huán)引用報(bào)錯(cuò)return reject(new TypeError('循環(huán)引用'));}let called = false; // 避免多次調(diào)用// 如果x是一個(gè)promise對(duì)象 （該判斷和下面 判斷是不是thenable對(duì)象重復(fù) 所以可有可無）if (x instanceof Promise) { // 獲得它的終值 繼續(xù)resolveif (x.status === PENDING) { // 如果為等待態(tài)需等待直至 x 被執(zhí)行或拒絕 并解析y值x.then(y => {resolvePromise(promise2, y, resolve, reject);}, reason => {reject(reason);});} else { // 如果 x 已經(jīng)處于執(zhí)行態(tài)/拒絕態(tài)(值已經(jīng)被解析為普通值)，用相同的值執(zhí)行傳遞下去 promisex.then(resolve, reject);}// 如果 x 為對(duì)象或者函數(shù)} else if (x != null && ((typeof x === 'object') || (typeof x === 'function'))) {try { // 是否是thenable對(duì)象（具有then方法的對(duì)象/函數(shù)）let then = x.then;if (typeof then === 'function') {then.call(x, y => {if(called) return;called = true;resolvePromise(promise2, y, resolve, reject);}, reason => {if(called) return;called = true;reject(reason);})} else { // 說明是一個(gè)普通對(duì)象/函數(shù)resolve(x);}} catch(e) {if(called) return;called = true;reject(e);}} else {resolve(x);} }/*** [注冊(cè)fulfilled狀態(tài)/rejected狀態(tài)對(duì)應(yīng)的回調(diào)函數(shù)]* @param {function} onFulfilled fulfilled狀態(tài)時(shí) 執(zhí)行的函數(shù)* @param {function} onRejected rejected狀態(tài)時(shí) 執(zhí)行的函數(shù)* @return {function} newPromsie 返回一個(gè)新的promise對(duì)象*/ Promise.prototype.then = function(onFulfilled, onRejected) {const that = this;let newPromise;// 處理參數(shù)默認(rèn)值 保證參數(shù)后續(xù)能夠繼續(xù)執(zhí)行onFulfilled =typeof onFulfilled === "function" ? onFulfilled : value => value;onRejected =typeof onRejected === "function" ? onRejected : reason => {throw reason;};// then里面的FULFILLED/REJECTED狀態(tài)時(shí) 為什么要加setTimeout ?// 原因:// 其一 2.2.4規(guī)范 要確保 onFulfilled 和 onRejected 方法異步執(zhí)行(且應(yīng)該在 then 方法被調(diào)用的那一輪事件循環(huán)之后的新執(zhí)行棧中執(zhí)行) 所以要在resolve里加上setTimeout// 其二 2.2.6規(guī)范 對(duì)于一個(gè)promise，它的then方法可以調(diào)用多次.（當(dāng)在其他程序中多次調(diào)用同一個(gè)promise的then時(shí) 由于之前狀態(tài)已經(jīng)為FULFILLED/REJECTED狀態(tài)，則會(huì)走的下面邏輯),所以要確保為FULFILLED/REJECTED狀態(tài)后 也要異步執(zhí)行onFulfilled/onRejected// 其二 2.2.6規(guī)范 也是resolve函數(shù)里加setTimeout的原因// 總之都是 讓then方法異步執(zhí)行 也就是確保onFulfilled/onRejected異步執(zhí)行// 如下面這種情景 多次調(diào)用p1.then// p1.then((value) => { // 此時(shí)p1.status 由pedding狀態(tài) => fulfilled狀態(tài)// console.log(value); // resolve// // console.log(p1.status); // fulfilled// p1.then(value => { // 再次p1.then 這時(shí)已經(jīng)為fulfilled狀態(tài) 走的是fulfilled狀態(tài)判斷里的邏輯 所以我們也要確保判斷里面onFuilled異步執(zhí)行// console.log(value); // 'resolve'// });// console.log('當(dāng)前執(zhí)行棧中同步代碼');// })// console.log('全局執(zhí)行棧中同步代碼');//if (that.status === FULFILLED) { // 成功態(tài)return newPromise = new Promise((resolve, reject) => {setTimeout(() => {try{let x = onFulfilled(that.value);resolvePromise(newPromise, x, resolve, reject); // 新的promise resolve 上一個(gè)onFulfilled的返回值} catch(e) {reject(e); // 捕獲前面onFulfilled中拋出的異常 then(onFulfilled, onRejected);}});})}if (that.status === REJECTED) { // 失敗態(tài)return newPromise = new Promise((resolve, reject) => {setTimeout(() => {try {let x = onRejected(that.reason);resolvePromise(newPromise, x, resolve, reject);} catch(e) {reject(e);}});});}if (that.status === PENDING) { // 等待態(tài)// 當(dāng)異步調(diào)用resolve/rejected時(shí) 將onFulfilled/onRejected收集暫存到集合中return newPromise = new Promise((resolve, reject) => {that.onFulfilledCallbacks.push((value) => {try {let x = onFulfilled(value);resolvePromise(newPromise, x, resolve, reject);} catch(e) {reject(e);}});that.onRejectedCallbacks.push((reason) => {try {let x = onRejected(reason);resolvePromise(newPromise, x, resolve, reject);} catch(e) {reject(e);}});});} };/*** Promise.all Promise進(jìn)行并行處理* 參數(shù): promise對(duì)象組成的數(shù)組作為參數(shù)* 返回值: 返回一個(gè)Promise實(shí)例* 當(dāng)這個(gè)數(shù)組里的所有promise對(duì)象全部變?yōu)閞esolve狀態(tài)的時(shí)候，才會(huì)resolve。*/ Promise.all = function(promises) {return new Promise((resolve, reject) => {let done = gen(promises.length, resolve);promises.forEach((promise, index) => {promise.then((value) => {done(index, value)}, reject)})}) }function gen(length, resolve) {let count = 0;let values = [];return function(i, value) {values[i] = value;if (++count === length) {console.log(values);resolve(values);}} }/*** Promise.race* 參數(shù): 接收 promise對(duì)象組成的數(shù)組作為參數(shù)* 返回值: 返回一個(gè)Promise實(shí)例* 只要有一個(gè)promise對(duì)象進(jìn)入 FulFilled 或者 Rejected 狀態(tài)的話，就會(huì)繼續(xù)進(jìn)行后面的處理(取決于哪一個(gè)更快)*/ Promise.race = function(promises) {return new Promise((resolve, reject) => {promises.forEach((promise, index) => {promise.then(resolve, reject);});}); }// 用于promise方法鏈時(shí) 捕獲前面onFulfilled/onRejected拋出的異常 Promise.prototype.catch = function(onRejected) {return this.then(null, onRejected); }Promise.resolve = function (value) {return new Promise(resolve => {resolve(value);}); }Promise.reject = function (reason) {return new Promise((resolve, reject) => {reject(reason);}); }/*** 基于Promise實(shí)現(xiàn)Deferred的* Deferred和Promise的關(guān)系* - Deferred 擁有 Promise* - Deferred 具備對(duì) Promise的狀態(tài)進(jìn)行操作的特權(quán)方法（resolve reject）**參考jQuery.Deferred*url: http://api.jquery.com/category/deferred-object/*/ Promise.deferred = function() { // 延遲對(duì)象let defer = {};defer.promise = new Promise((resolve, reject) => {defer.resolve = resolve;defer.reject = reject;});return defer; }/*** Promise/A+規(guī)范測(cè)試* npm i -g promises-aplus-tests* promises-aplus-tests Promise.js*/try {module.exports = Promise } catch (e) { }復(fù)制代碼

Promise測(cè)試

npm i -g promises-aplus-tests promises-aplus-tests Promise.js 復(fù)制代碼

如何主動(dòng)終止Promise調(diào)用鏈

const p1 = new Promise((resolve, reject) => {setTimeout(() => { // 異步操作resolve('start')}, 1000); });p1.then((result) => {console.log('a', result); return Promise.reject('中斷后續(xù)調(diào)用'); // 此時(shí)rejected的狀態(tài)將直接跳到catch里，剩下的調(diào)用不會(huì)再繼續(xù) }).then(result => {console.log('b', result); }).then(result => {console.log('c', result); }).catch(err => {console.log(err); });// a start // 中斷后續(xù)調(diào)用 復(fù)制代碼

相關(guān)知識(shí)參考資料

• ES6-promise
• Promises/A+規(guī)范-英文
• Promises/A+規(guī)范-翻譯1
• Promises/A+規(guī)范-翻譯-推薦
• JS執(zhí)行棧
• Javascript異步編程的4種方法

創(chuàng)作挑戰(zhàn)賽新人創(chuàng)作獎(jiǎng)勵(lì)來咯，堅(jiān)持創(chuàng)作打卡瓜分現(xiàn)金大獎(jiǎng)

總結(jié)

以上是生活随笔為你收集整理的Promise 原理解析与实现(遵循Promise/A+规范)的全部?jī)?nèi)容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網(wǎng)站內(nèi)容還不錯(cuò)，歡迎將生活随笔推薦給好友。