點我看看~

前言：可能存在闡述不準確之處，歡迎指正～

Promise在long time ago就活躍于Javascript社區，受到開發者歡迎，只不過到近幾年才被納入ECMA規范。

我們為什么要使用Promsie？

因為：

我們不希望，過了幾個月之后，代碼只有上帝才看得懂；
我們不希望，回調代碼越寫越往右，只能換更大的顯示器看；
我們希望，哪怕過了很久，代碼依舊邏輯清晰，看懂不費吹灰之力；
我們希望，能夠用自己的雙手，控制住異步流的動向，就像同步代碼一樣；
有逼格...
(歡迎補充)

話不多說，上源碼（采用es6/es7， 創建一個Commitment類模仿Promise）：：

class Commitment {constructor (executor) {this.status = "pending";this.value = void(0);this.reason = void(0);this.onResolvedCallbacks = [];this.onRejectedCallbacks = [];const resolve = (value) => {if (value instanceof Promise) {return value.then(resolve, reject)}setTimeout(() => {if (this.status === "pending") {this.status = "resolved";this.value = value;this.onResolvedCallbacks.forEach(cb => cb(value));}})}const reject = (reason) => {setTimeout(() => {if (this.status === "pending") {this.status = "rejected";this.reason = reason;this.onRejectedCallbacks.forEach(cb => cb(reason));} })}try {executor(resolve, reject)} catch (e) {reject(e)}}then (onFulfilled, onRejected) {onFulfilled = typeof onFulfilled === "function" ? onFulfilled : value => value;onRejected = typeof onRejected === "function" ? onRejected : reason => {throw reason};const resolveCommitment = (Commitment2, x, resolve, reject) => {if (Commitment2 === x) {return reject(new TypeError("A promise cannot be resolved with itself"))}let then, called;if (x !== null && (typeof x === "object" || typeof x === "function")) {try {then = x.then;if (typeof then === "function") {then.call(x, y => {if (called) returncalled = true;resolveCommitment(Commitment2, y, resolve, reject);}, r => {if (called) returncalled = true;reject(r)})} else {resolve(x)}} catch (e) {if (called) returncalled = true;reject(e)}} else {resolve(x)}}let Commitment2, x;if (this.status === "resolved") {Commitment2 = new Commitment((resolve, reject) => {setTimeout(() => {try {x = onFulfilled(this.value);resolveCommitment(Commitment2, x, resolve, reject)} catch (e) {reject(e)} });})}if (this.status === "rejected") {Commitment2 = new Commitment((resolve, reject) => {setTimeout(() => {try {x = onRejected(this.reason);resolveCommitment(Commitment2, x, resolve, reject)} catch (e) {reject(e)}})})}if (this.status === "pending") {Commitment2 = new Commitment((resolve, reject) => {this.onResolvedCallbacks.push((value)=> {try {x = onFulfilled(value);resolveCommitment(Commitment2, x, resolve, reject)} catch (e) {reject(e)} })this.onRejectedCallbacks.push((reason) => {try {x = onRejected(reason);resolveCommitment(Commitment2, x, resolve, reject)} catch (e) {reject(e)}})})}return Commitment2} }

總結

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