let p2 = new Promise((reslove, reject) => {reslove(1)})console.log(p2.then(res => {alert(res)}))

/***   promise*   异步编程的一种解决方案。** */const promise = new Promise(function (reslove, reject) {console.log('开始')setTimeout(function () {reslove(2)}, 3000)})promise.then(function (value) {return 3;}).then(function (value) {})//let p = new Promise((reslove, reject) => {setTimeout(()=>{let a = Math.random();if (a>0.5) {//reslove(a); // 这里说白了是就是调用下面的then中的函数 areslove.call(null, a) // 这样也行} else {reject(a) // 调用b}reject(a) // 调用b}, 1000)})p.then(res => { // 假设这是函数areturn res;}, rej => { // 函数breturn rej;}).then(success => { // 链式调用的重点是前边的then必须return 否则 为undefinedreturn success;}, fail => {return fail;})// then() then方法是挂在Promise.prototype上的console.log(Promise.prototype) // Promise {constructor: ƒ, then: ƒ, catch: ƒ, finally: ƒ, Symbol(Symbol.toStringTag): "Promise"}// catch（） 相当于 then(null, reject)p.then(rel => {console.log(rel)return rel;},rej => {console.log(rej + '小')return rej;}).then(rel => { // 前一个then返回成功或失败都在这里  因为执行catch()后返回一个新的实例该实例该实例执行完之后会变为reslove ，因此后边的catch始终执行不到console.log(rel)alert(2)}).catch(rej => { // 这句是走不到的console.log(rej)alert(1)console.log('二小')})// finally() 状态成功或失败 都会执行此操作let p1 = new Promise((reslove, reject) => {console.log(this) // window 箭头函数本身没有this 因此this指向定义箭头函数时的上下文reslove(1);})p1.then(res => {console.log('success')}).finally(res => {console.log('finally')})// all() 所有实例的状态都改变完才改变let p2 = new Promise((reslove, reject) => {setTimeout(() => {alert(1)reslove(1)} ,1000)})let p3 = new Promise((reslove, reject) => {setTimeout(() => {alert(2)reslove(2)} ,2000)})let p4 = new Promise((reslove, reject) => {setTimeout(() => {alert(3)reject(3)} ,3000)})Promise.all([p2, p3]).then(res => {console.log(res) // [1, 2]})Promise.all([p2, p3, p4]).then(res => {console.log(res)}).catch(rej => {console.log(rej) // 3  只要有一个reject 就走这里 不走上边的then 返回的永远是第一个reject的promise实例})// race()Promise.race([p2, p3]).then(res => {console.log(res) // 1})

// 实现 promisefunction MyPromise(fn){ // fn 为 new Promise(fn)let _this = this;_this.status = 'pending';_this.resloveValue = null; // 成功时的数据_this.rejectValue = null; // 失败时的数据_this.resloveFnCallbacks = [];_this.rejectFnCallbacks = [];function reslove(value) {if (_this.status == 'pending') {_this.status = 'resloved';_this.resloveValue = value;_this.resloveFnCallbacks.forEach(fn => fn())}}function reject(value) {if (_this.status == 'pending') {_this.status = 'rejected';_this.rejectValue = value;_this.rejectFnCallbacks.forEach(fn => fn())}}// 捕获异常try{fn(reslove, reject)} catch (e) {reject(e)}}// then() 应该接受两个参数 成功 或 失败的 回调MyPromise.prototype.then = function (resloveFn, rejectFn) {let _this = this;let promise2; // then 返回的promise// 参数非函数转为函数resloveFn = typeof resloveFn === 'function'? resloveFn: (function (value) {return value;})rejectFn = typeof rejectFn === 'function'? rejectFn: function (err) {throw err;}if(this.status == 'resloved') { // 成功alert(3)promise2 =  new MyPromise((reslove, reject) => {setTimeout(() => {let x = resloveFn(this.resloveValue);resolvePromise(promise2, x, reslove, reject);})})}if(this.status == 'rejected') { // 失败rejectFn(this.rejectValue)}if (this.status == 'pending') {this.resloveFnCallbacks.push(function(){resloveFn(_this.resloveValue)})this.rejectFnCallbacks.push(function(){rejectFn(_this.rejectValue)})}return promise2;// resolvePromisefunction resolvePromise(promise2, x, reslove, reject) {let called; // 是否调用过// 判断上一次then返回的值 实质上下一个then只需要一个值便可以，所以我们就是要将上一个的peomise中的值提取出来// then 返回promise只是为了链式调用，then本身需要一个实际的值，并不需要promiseif (x != null && (typeof x === 'object' || typeof x === 'function')) {try {let then = x.then; // 保存x的then方法 这种是x是对象if (typeof then === 'function') {then.call(x, function (y) {if (called) return;called = true;resolvePromise(promise2, y, reslove, reject)}, function (rej) {if (called) return;called = true;reject(rej)})} else { // x 是普通对象直接返回就行reslove(x);}} catch (e) {if (called) return;called = true;reject(e)}} else { // x 是普通值reslove(x)}}}// 将promiselet p6 = new MyPromise(function(reslove, reject) {reslove(2)})p6.then(function(res){return res;}).then(function(res){alert(res)})