关于javascript:javascript-Promise底层实现

class Promise {  constructor(handle) {    if (typeof handle !== "function") {      throw new Error("promise must accept a function as a parameter");    }    // 定义三种状态配置    this.STATUS = {      PENDING: "pending",      FULFILLED: "fulfilled",      REJECTED: "rejected"    };    // 初始化状态    this._status = this.STATUS.PENDING;    // 初始化值    this._value = undefined;    // 胜利状态须要执行的工作队列    this._fulfilledQueues = [];    // 失败状态须要执行的工作队列    this._rejectedQueues = [];    // 执行handle    try {      handle(this._resolve.bind(this), this._reject.bind(this));    } catch (err) {      this._reject(err);    }  }  // Object.prototype.toString.call(promiseObj) === "[object Promise]"  get [Symbol.toStringTag](){      return "Promise"  }  _runMicroTask(callback) {    // 应用浏览器MutationObserver WEB.API实现then办法的微工作机制    let count = 0;    const observer = new MutationObserver(callback);    // 创立文本节点，节约资源    const textNode = document.createTextNode(String(count));    observer.observe(textNode, {      // 当文本扭转时触发回调      characterData: true    });    // 扭转文本，回调callback触发    textNode.data = String(++count);  }  _resolve(val) {    if (this._status !== this.STATUS.PENDING) return;    const onFulfilled = v => {      this._status = this.STATUS.FULFILLED;      this._value = v;      let cb;      while ((cb = this._fulfilledQueues.shift())) {        cb(v);      }    };    const onRejected = err => {      this._status = this.STATUS.REJECTED;      this._value = err;      let cb;      while ((cb = this._rejectedQueues.shift())) {        cb(err);      }    };    // 顺次执行胜利队列中的函数，并清空队列    const run = () => {      if (val instanceof Promise) {        // 如果入参是一个Promise，则val的status及value值决定了以后promise实例的status和value值        val.then(onFulfilled, onRejected);      } else {        onFulfilled(val);      }    };    this._runMicroTask(run);  }  _reject(val) {    if (this._status !== this.STATUS.PENDING) return;    // 顺次执行失败队列中的函数，并清空队列    const run = () => {      this._status = this.STATUS.REJECTED;      this._value = val;      let cb;      while ((cb = this._rejectedQueues.shift())) {        cb(val);      }    };    this._runMicroTask(run);  }  then(onFulfilled, onRejected) {    // then反对链式调用，返回一个新的promise    return new Promise((resolve, reject) => {      const handleResolve = value => {        try {          if (typeof onFulfilled !== "function") {            // 如果onFulfilled不是函数，则间接返回以后promise的value值            resolve(value);          } else {            const res = onFulfilled(value);            // Promise.prototype 是否存在于res的原型链上，是则阐明res是Promise实例            if (res instanceof Promise) {              res.then(resolve, reject);            } else {              resolve(res);            }          }        } catch (err) {          reject(err);        }      };      const handleReject = value => {        try {          if (typeof onRejected !== "function") {            // 如果onFulfilled不是函数，则间接返回以后promise的value值            resolve(value);          } else {            const res = onRejected(value);            if (res instanceof Promise) {              res.then(resolve, reject);            } else {              resolve(res);            }          }        } catch (err) {          reject(err);        }      };      switch (this._status) {        case this.STATUS.PENDING:          this._fulfilledQueues.push(handleResolve);          this._rejectedQueues.push(handleReject);          break;        case this.STATUS.FULFILLED:          handleResolve(this._value);          break;        case this.STATUS.REJECTED:          handleReject(this._value);          break;      }    });  }  catch(onRejected) {    return this.then(null, onRejected);  }  // 增加动态resolve办法  static resolve(value) {    // 如果参数是MyPromise实例，间接返回这个实例    if (value instanceof Promise) return value;    return new Promise(resolve => resolve(value));  }  // 增加动态reject办法  static reject(err) {    return new Promise((resolve, reject) => reject(err));  }  // 增加动态all办法,所有promise入参状态产生扭转后才扭转状态  static all(list) {    return new Promise((resolve, reject) => {      const length = list.length;      let count = 0;      let values = [];      // // entries返回数组的键值对[[index, value]...]      // for (let [i, p] of list.entries()) {      //      // }      for (let i = 0; i < length; i++) {        const promise = list[i];        promise.then(          res => {            // promise有可能异步执行resolve，不能用push            values[i] = res;            count++;            if (count === length) {              resolve(values);            }          },          err => {            // 只有子元素promise中有任何一个reject，则返回的promise rejected            reject(err);          }        );      }    });  }  // 增加动态race办法  static race(list) {    return new Promise((resolve, reject) => {      for (let p of list) {        // 只有有一个实例率先扭转状态，新的MyPromise的状态就跟着扭转        this.resolve(p).then(          res => {            resolve(res);          },          err => {            reject(err);          }        );      }    });  }  /*finally() 办法返回一个Promise。    在promise完结时，无论后果是fulfilled或者是rejected，都会执行指定的回调函数。    这为在Promise是否胜利实现后都须要执行的代码提供了一种形式。    这防止了同样的语句须要在then()和catch()中各写一次的状况。*/  finally(handle) {    return this.then(      value => {        handle();        return Promise.resolve(value);      },      err => {        handle();        return Promise.reject(err);      }    );  }}// console.log(Promise.resolve)// // new Promise((res, rej) => setTimeout(() => rej(777)), 1000)// //     .then(res => console.log({res}), err => console.log(1, err))// //     .then(res => console.log({res}), err => console.log(2, err))//// console.log(Promise.resolve(Promise.resolve(222)))////// // new Promise((res, rej) => rej(666))// //     .then(res => console.log({res}), err => new Promise(res => setTimeout(() => res (123),3000)))// //     .then(res => console.log({then2: res}), err => console.log(2, err))//// new Promise((res, rej) => res(new Promise((r, err) => err(857))))//     .then(null, e => new Promise(res => res(111)))//     .then(a => console.log({a})).catch(cat => console.log({cat}))console.log("哇哈哈");setTimeout(() => {  console.log("setTimeout");}, 0);new Promise((res, rej) => {  rej(111);})  .finally(() => {    console.log("执行finally");  })  .then(null, err => console.log({ err }));console.log("666");if (window) {  console.log("you dinwow");}