Promise源码实现

const ENUM = {    PENDING: 'PENDING',    FULFILLED: 'FULFILLED',    REJECTED: 'REJECTED'}// 需要兼容其他人的promiseconst resolvePromise = (x, promise2, resolve, reject) => {    if (x === promise2) {        reject(new TypeError(`TypeError: Chaining cycle detected for promise #<Promise>`))    }    if ((typeof x === 'object' && x !== null) || typeof x === 'function') {        let called;        try {            let then = x.then;             if (typeof then == 'function') {                then.call(x, y => {                     if (called) return;                    called = true;                    resolvePromise(y, promise2, resolve, reject);                }, r => {                    if (called) return;                    called = true;                    reject(r);                })            } else {                resolve(x); // {a:1}            }        } catch (e) {            if (called) return;            called = true;            reject(e);        }    } else {        resolve(x);     }}class Promise {    constructor(executor) {        this.status = ENUM.PENDING;        this.value = undefined; // 实例上的属性 每次new 都会创建一个新的        this.reason = undefined;        // 需要创建成功的队列 失败的队列，分别存放        this.onResolvedCallbacks = [];        this.onRejectedCallbacks = [];        const resolve = (value) => {            // 如果value 是一个promise，那我们的库中应该也要实现一个递归解析            if(value instanceof Promise){                // 递归解析                 return value.then(resolve,reject)            }            if (this.status === ENUM.PENDING) {                this.status = ENUM.FULFILLED;                this.value = value; // 先赋予的值 再循环                this.onResolvedCallbacks.forEach(fn => fn())            }        }        const reject = (reason) => {            if (this.status === ENUM.PENDING) {                this.status = ENUM.REJECTED;                this.reason = reason;                this.onRejectedCallbacks.forEach(fn => fn())            }        }        try {            executor(resolve, reject);        } catch (e) {            reject(e)        }    }    then(onFulfilled, onRejected) {        onFulfilled = typeof onFulfilled == 'function' ? onFulfilled : v => v;        onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };        // 调用then 方法 创建一个新的promise        let promise2 = new Promise((resolve, reject) => {            // 我需要根据 x 的状况来判断是调用resolve还是reject            if (this.status == ENUM.FULFILLED) {                // todo...                setTimeout(() => {                    try {                        let x = onFulfilled(this.value);                        // 解析promise                        resolvePromise(x, promise2, resolve, reject)                    } catch (e) {                        reject(e);                    }                }, 0);            }            if (this.status == ENUM.REJECTED) {                // todo...                setTimeout(() => {                    try {                        let x = onRejected(this.reason);                        resolvePromise(x, promise2, resolve, reject)                    } catch (e) {                        reject(e);                    }                }, 0);            }            if (this.status == ENUM.PENDING) {                // 用户还没有调用resolve或者reject方法                this.onResolvedCallbacks.push(() => {                    // todo...                    setTimeout(() => {                        try {                            let x = onFulfilled(this.value);                            resolvePromise(x, promise2, resolve, reject)                        } catch (e) {                            reject(e);                        }                    }, 0);                });                this.onRejectedCallbacks.push(() => {                    // todo...                    setTimeout(() => {                        try {                            let x = onRejected(this.reason);                            resolvePromise(x, promise2, resolve, reject)                        } catch (e) {                            reject(e);                        }                    }, 0);                })            }        });        return promise2;    }    catch(errCallback){        return this.then(null,errCallback)    }    static resolve(val){        return new Promise((resolve,reject)=>{            resolve(val);        })    }    static reject(reason){        return new Promise((resolve,reject)=>{            reject(reason);        })    }}// 产生延迟对象Promise.prototype.finally = function(callback) {    return this.then((value) => {        return Promise.resolve(callback()).then(() => value)    }, (err) => {        return Promise.resolve(callback()).then(() => { throw err })    });}Promise.all = function(values) {    return new Promise((resolve, reject) => {        let resultArr = [];        let orderIndex = 0;        const processResultByKey = (value, index) => {            resultArr[index] = value;            if (++orderIndex === values.length) {                resolve(resultArr)            }        }        for (let i = 0; i < values.length; i++) {            let value = values[i];            if (value && typeof value.then === 'function') {                value.then((value) => {                    processResultByKey(value, i);                }, reject);            } else {                processResultByKey(value, i);            }        }    });}Promise.race = function(promises) {    return new Promise((resolve, reject) => {        // 一起执行就是for循环        for (let i = 0; i < promises.length; i++) {            let val = promises[i];            if (val && typeof val.then === 'function') {                val.then(resolve, reject); // value.then(resolve,reject)            } else { // 普通值                resolve(val)            }        }    });}