我们工作中免不了运用promise用来解决异步回调问题。平时用的很多库或者插件都运用了promise 例如axios、fetch等等。但是你知道promise是咋写出来的呢？
别怕～这里有本promisesA+规范，便宜点10元卖给你
1、Promise 的声明
首先呢，promise肯定是一个类，我们就用class来声明。
由于
new Promise((resolve, reject)=>{})，所以传入一个参数（函数），秘籍里叫他executor，传入就执行。executor里面有两个参数，一个叫resolve（成功），一个叫reject（失败）。
由于resolve和reject可执行，所以都是函数，我们用let声明。
class Promise{ // 构造器 constructor(executor){ // 成功 let resolve = () => { }; // 失败 let reject = () => { }; // 立即执行 executor(resolve, reject); } }解决基本状态
秘籍对Promise有规定：
Promise存在三个状态（state）pending、fulfilled、rejected
pending（等待态）为初始态，并可以转化为fulfilled（成功态）和rejected（失败态）
成功时，不可转为其他状态，且必须有一个不可改变的值（value）
失败时，不可转为其他状态，且必须有一个不可改变的原因（reason）
new Promise((resolve, reject)=>{resolve(value)})resolve为成功，接收参数value，状态改变为fulfilled，不可再次改变。new Promise((resolve, reject)=>{reject(reason)})reject为失败，接收参数reason，状态改变为rejected，不可再次改变。若是executor函数报错 直接执行reject();
于是乎，我们获得以下代码
class Promise{ constructor(executor){ // 初始化state为等待态 this.state = 'pending'; // 成功的值 this.value = undefined; // 失败的原因 this.reason = undefined; let resolve = value => { // state改变,resolve调用就会失败 if (this.state === 'pending') { // resolve调用后，state转化为成功态 this.state = 'fulfilled'; // 储存成功的值 this.value = value; } }; let reject = reason => { // state改变,reject调用就会失败 if (this.state === 'pending') { // reject调用后，state转化为失败态 this.state = 'rejected'; // 储存失败的原因 this.reason = reason; } }; // 如果executor执行报错，直接执行reject try{ executor(resolve, reject); } catch (err) { reject(err); } } }then方法
秘籍规定:Promise有一个叫做then的方法，里面有两个参数：onFulfilled,onRejected,成功有成功的值，失败有失败的原因
当状态state为fulfilled，则执行onFulfilled，传入this.value。当状态state为rejected，则执行onRejected，传入this.value
onFulfilled,onRejected如果他们是函数，则必须分别在fulfilled，rejected后被调用，value或reason依次作为他们的第一个参数
class Promise{ constructor(executor){...} // then 方法 有两个参数onFulfilled onRejected then(onFulfilled,onRejected) { // 状态为fulfilled，执行onFulfilled，传入成功的值 if (this.state === 'fulfilled') { onFulfilled(this.value); }; // 状态为rejected，执行onRejected，传入失败的原因 if (this.state === 'rejected') { onRejected(this.reason); }; } }这下武学初成，可以对付对付江湖小杂毛了，但是对于带setTimeout的江洋大盗还是没辙。
解决异步实现
现在基本可以实现简单的同步代码，但是当resolve在setTomeout内执行，then时state还是pending等待状态 我们就需要在then调用的时候，将成功和失败存到各自的数组，一旦reject或者resolve，就调用它们
类似于发布订阅，先将then里面的两个函数储存起来，由于一个promise可以有多个then，所以存在同一个数组内。
// 多个then的情况 let p = new Promise(); p.then(); p.then();
成功或者失败时，forEach调用它们
class Promise{ constructor(executor){ this.state = 'pending'; this.value = undefined; this.reason = undefined; // 成功存放的数组 this.onResolvedCallbacks = []; // 失败存放法数组 this.onRejectedCallbacks = []; let resolve = value => { if (this.state === 'pending') { this.state = 'fulfilled'; this.value = value; // 一旦resolve执行，调用成功数组的函数 this.onResolvedCallbacks.forEach(fn=>fn()); } }; let reject = reason => { if (this.state === 'pending') { this.state = 'rejected'; this.reason = reason; // 一旦reject执行，调用失败数组的函数 this.onRejectedCallbacks.forEach(fn=>fn()); } }; try{ executor(resolve, reject); } catch (err) { reject(err); } } then(onFulfilled,onRejected) { if (this.state === 'fulfilled') { onFulfilled(this.value); }; if (this.state === 'rejected') { onRejected(this.reason); }; // 当状态state为pending时 if (this.state === 'pending') { // onFulfilled传入到成功数组 this.onResolvedCallbacks.push(()=>{ onFulfilled(this.value); }) // onRejected传入到失败数组 this.onRejectedCallbacks.push(()=>{ onRejected(this.reason); }) } } }解决链式调用
我门常常用到new Promise().then().then(),这就是链式调用，用来解决回调地狱
1、为了达成链式，我们默认在第一个then里返回一个promise。秘籍规定了一种方法，就是在then里面返回一个新的promise,称为promise2：promise2 = new Promise((resolve, reject)=>{})
将这个promise2返回的值传递到下一个then中
如果返回一个普通的值，则将普通的值传递给下一个then中
2、当我们在第一个then中return了一个参数（参数未知，需判断）。这个return出来的新的promise就是onFulfilled()或onRejected()的值
秘籍则规定onFulfilled()或onRejected()的值，即第一个then返回的值，叫做x，判断x的函数叫做resolvePromise
首先，要看x是不是promise。
如果是promise，则取它的结果，作为新的promise2成功的结果
如果是普通值，直接作为promise2成功的结果
所以要比较x和promise2
resolvePromise的参数有promise2（默认返回的promise）、x（我们自己
return的对象）、resolve、rejectresolve和reject是promise2的
class Promise{ constructor(executor){ this.state = 'pending'; this.value = undefined; this.reason = undefined; this.onResolvedCallbacks = []; this.onRejectedCallbacks = []; let resolve = value => { if (this.state === 'pending') { this.state = 'fulfilled'; this.value = value; this.onResolvedCallbacks.forEach(fn=>fn()); } }; let reject = reason => { if (this.state === 'pending') { this.state = 'rejected'; this.reason = reason; this.onRejectedCallbacks.forEach(fn=>fn()); } }; try{ executor(resolve, reject); } catch (err) { reject(err); } } then(onFulfilled,onRejected) { // 声明返回的promise2 let promise2 = new Promise((resolve, reject)=>{ if (this.state === 'fulfilled') { let x = onFulfilled(this.value); // resolvePromise函数，处理自己return的promise和默认的promise2的关系 resolvePromise(promise2, x, resolve, reject); }; if (this.state === 'rejected') { let x = onRejected(this.reason); resolvePromise(promise2, x, resolve, reject); }; if (this.state === 'pending') { this.onResolvedCallbacks.push(()=>{ let x = onFulfilled(this.value); resolvePromise(promise2, x, resolve, reject); }) this.onRejectedCallbacks.push(()=>{ let x = onRejected(this.reason); resolvePromise(promise2, x, resolve, reject); }) } }); // 返回promise，完成链式 return promise2; } }完成resolvePromise函数
秘籍规定了一段代码，让不同的promise代码互相套用，叫做resolvePromise
如果 x === promise2，则是会造成循环引用，自己等待自己完成，则报“循环引用”错误
let p = new Promise(resolve => { resolve(0); }); var p2 = p.then(data => { // 循环引用，自己等待自己完成，一辈子完不成 return p2; })1、判断x
Otherwise, if x is an object or function,Let then be x.then
x 不能是null
x 是普通值 直接resolve(x)
x 是对象或者函数（包括promise），
let then = x.then2、当x是对象或者函数（默认promise）声明了then
如果取then报错，则走reject()
如果then是个函数，则用call执行then，第一个参数是this，后面是成功的回调和失败的回调
如果成功的回调还是pormise，就递归继续解析 3、成功和失败只能调用一个 所以设定一个called来防止多次调用
function resolvePromise(promise2, x, resolve, reject){ // 循环引用报错 if(x === promise2){ // reject报错 return reject(new TypeError('Chaining cycle detected for promise')); } // 防止多次调用 let called; // x不是null 且x是对象或者函数 if (x != null && (typeof x === 'object' || typeof x === 'function')) { try { // A+规定，声明then = x的then方法 let then = x.then; // 如果then是函数，就默认是promise了 if (typeof then === 'function') { // 就让then执行 第一个参数是this 后面是成功的回调 和 失败的回调 then.call(x, y => { // 成功和失败只能调用一个 if (called) return; called = true; // resolve的结果依旧是promise 那就继续解析 resolvePromise(promise2, y, resolve, reject); }, err => { // 成功和失败只能调用一个 if (called) return; called = true; reject(err);// 失败了就失败了 }) } else { resolve(x); // 直接成功即可 } } catch (e) { // 也属于失败 if (called) return; called = true; // 取then出错了那就不要在继续执行了 reject(e); } } else { resolve(x); } }解决其他问题
1、秘籍规定onFulfilled,onRejected都是可选参数，如果他们不是函数，必须被忽略
onFulfilled返回一个普通的值，成功时直接等于
value => valueonRejected返回一个普通的值，失败时如果直接等于 value => value，则会跑到下一个then中的onFulfilled中，所以直接扔出一个错误
reason => throw err2、秘籍规定onFulfilled或onRejected不能同步被调用，必须异步调用。我们就用setTimeout解决异步问题如果onFulfilled或onRejected报错，则直接返回reject()
class Promise{ constructor(executor){ this.state = 'pending'; this.value = undefined; this.reason = undefined; this.onResolvedCallbacks = []; this.onRejectedCallbacks = []; let resolve = value => { if (this.state === 'pending') { this.state = 'fulfilled'; this.value = value; this.onResolvedCallbacks.forEach(fn=>fn()); } }; let reject = reason => { if (this.state === 'pending') { this.state = 'rejected'; this.reason = reason; this.onRejectedCallbacks.forEach(fn=>fn()); } }; try{ executor(resolve, reject); } catch (err) { reject(err); } } then(onFulfilled,onRejected) { // onFulfilled如果不是函数，就忽略onFulfilled，直接返回value onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value; // onRejected如果不是函数，就忽略onRejected，直接扔出错误 onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err }; let promise2 = new Promise((resolve, reject) => { if (this.state === 'fulfilled') { // 异步 setTimeout(() => { try { let x = onFulfilled(this.value); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }, 0); }; if (this.state === 'rejected') { // 异步 setTimeout(() => { // 如果报错 try { let x = onRejected(this.reason); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }, 0); }; if (this.state === 'pending') { this.onResolvedCallbacks.push(() => { // 异步 setTimeout(() => { try { let x = onFulfilled(this.value); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }, 0); }); this.onRejectedCallbacks.push(() => { // 异步 setTimeout(() => { try { let x = onRejected(this.reason); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }, 0) }); }; }); // 返回promise，完成链式 return promise2; } }大功告成
class Promise{ constructor(executor){ this.state = 'pending'; this.value = undefined; this.reason = undefined; this.onResolvedCallbacks = []; this.onRejectedCallbacks = []; let resolve = value => { if (this.state === 'pending') { this.state = 'fulfilled'; this.value = value; this.onResolvedCallbacks.forEach(fn=>fn()); } }; let reject = reason => { if (this.state === 'pending') { this.state = 'rejected'; this.reason = reason; this.onRejectedCallbacks.forEach(fn=>fn()); } }; try{ executor(resolve, reject); } catch (err) { reject(err); } } then(onFulfilled,onRejected) { onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value; onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err }; let promise2 = new Promise((resolve, reject) => { if (this.state === 'fulfilled') { setTimeout(() => { try { let x = onFulfilled(this.value); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }, 0); }; if (this.state === 'rejected') { setTimeout(() => { try { let x = onRejected(this.reason); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }, 0); }; if (this.state === 'pending') { this.onResolvedCallbacks.push(() => { setTimeout(() => { try { let x = onFulfilled(this.value); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }, 0); }); this.onRejectedCallbacks.push(() => { setTimeout(() => { try { let x = onRejected(this.reason); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }, 0) }); }; }); return promise2; } catch(fn){ return this.then(null,fn); } } function resolvePromise(promise2, x, resolve, reject){ if(x === promise2){ return reject(new TypeError('Chaining cycle detected for promise')); } let called; if (x != null && (typeof x === 'object' || typeof x === 'function')) { try { let then = x.then; if (typeof then === 'function') { then.call(x, y => { if(called)return; called = true; resolvePromise(promise2, y, resolve, reject); }, err => { if(called)return; called = true; reject(err); }) } else { resolve(x); } } catch (e) { if(called)return; called = true; reject(e); } } else { resolve(x); } } //resolve方法 Promise.resolve = function(val){ return new Promise((resolve,reject)=>{ resolve(val) }); } //reject方法 Promise.reject = function(val){ return new Promise((resolve,reject)=>{ reject(val) }); } //race方法 Promise.race = function(promises){ return new Promise((resolve,reject)=>{ for(let i=0;i<promises.length;i++){ promises[i].then(resolve,reject) }; }) } //all方法(获取所有的promise，都执行then，把结果放到数组，一起返回) Promise.all = function(promises){ let arr = []; let i = 0; function processData(index,data){ arr[index] = data; i++; if(i == promises.length){ resolve(arr); }; }; return new Promise((resolve,reject)=>{ for(let i=0;i<promises.length;i++){ promises[i].then(data=>{ processData(i,data); },reject); }; }); }如何验证我们的promise是否正确
1、先在后面加上下述代码
2、npm 有一个promises-aplus-tests插件 npm i promises-aplus-tests -g 可以全局安装 mac用户最前面加上sudo
3、命令行 promises-aplus-tests [js文件名] 即可验证
// 目前是通过他测试 他会测试一个对象 // 语法糖 Promise.defer = Promise.deferred = function () { let dfd = {} dfd.promise = new Promise((resolve,reject)=>{ dfd.resolve = resolve; dfd.reject = reject; }); return dfd; } module.exports = Promise; //npm install promises-aplus-tests 用来测试自己的promise 符不符合promisesA+规范
作者：卡姆爱卡姆
链接：https://juejin.im/post/5b2f02cd5188252b937548ab
来源：掘金
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。