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手写PROMISE核心代码，提升JAVASCRIPT编程能力，加深JS异步任务理解，ES6必学知识

代码写在 HD.js 里

HD.js

class HD {
     static PENDING = 'pending'; // 准备  static FUlFILLED = 'fulfilled'; // 解决  static REJECTED = 'rejected'; // 拒绝  constructor(executor) {
       this.status = HD.PENDING;    this.value = null;    this.callbacks = [];    try {
         executor(this.resolve.bind(this), this.reject.bind(this));    } catch (error) {
         this.reject(error);    }  }  resolve(value) {
       // 异步特性    if (this.status == HD.PENDING) {
         this.status = HD.FUlFILLED;      this.value = value;      // 让与resolve同块的方法先执行      setTimeout(() => {
           // 处理准备时期 then 放入的回调        this.callbacks.forEach(callback => {
             callback['onFulfilled'](value);        })      })    }  }  reject(reason) {
       // 异步特性    if (this.status == HD.PENDING) {
         this.status = HD.REJECTED;      this.value = reason;      // 让与resolve同块的方法先执行      setTimeout(() => {
           // 处理准备时期 then 放入的回调        this.callbacks.forEach(callback => {
             callback['onRejected'](reason);        })      })    }  }  then(onFulfilled, onRejected) {
       // 没传resolve方法    if (typeof onFulfilled != 'function') {
         // then值穿透      onFulfilled = () => this.value;    }    // 没传reject方法    if (typeof onRejected != 'function') {
         // 没有catch异常      onRejected = function () {
           throw new Error("Unexpected end of input")      }    }    let promise = new HD((resolve, reject) => {
         // 准备      if (this.status == HD.PENDING) {
           this.callbacks.push({
             onFulfilled: value => {
               // 冗余代码的优化            HD.parse(promise, onFulfilled(value), resolve, reject)          },          onRejected: reason => {
               // 冗余代码的优化            HD.parse(promise, onRejected(reason), resolve, reject)          }        });      }      // 处理      else if (this.status == HD.FUlFILLED) {
           // 等待this.next赋值        setTimeout(() => {
             // 冗余代码的优化          HD.parse(promise, onFulfilled(this.value), resolve, reject)        })      }      // 拒绝      else if (this.status == HD.REJECTED) {
           // 等待this.next赋值        setTimeout(() => {
             // 冗余代码的优化          HD.parse(promise, onRejected(this.value), resolve, reject)        })      }    });    return promise;  }  static parse(promise, result, resolve, reject) {
       // 禁止同一个then中循环返回 promise    if (promise == result) {
         throw new TypeError('Chaining cycle detected')    }    try {
         // 设置结果值      if (result instanceof HD) {
           result.then(resolve, reject);      } else {
           resolve(result);      }    } catch (error) {
         reject(error);    }  }  static resolve(value) {
       return new HD((resolve, reject) => {
         HD.parse(null, value, resolve, reject);    })  }  static reject(value) {
       return new HD((resolve, reject) => {
         reject(value)    })  }  /**   * 全成功，返回全部返回值   * 有失败，返回第一个失败信息   * @param {HD} promises    */  static all(promises) {
       return new HD((resolve, reject) => {
         const values = [];      promises.forEach((promise) => {
           promise.then(          value => {
               values.push(value)            if (values.length == promises.length) {
                 resolve(values);            }          },          reason => {
               reject(reason)          }        )      })    })  }  /**   * race（竞赛）   * 返回第一个返回值   * @param {HD} promises    */  static race(promises) {
       return new HD((resolve, reject) => {
         promises.forEach(promise => {
           promise.then(          value => {
               return resolve(value);          },          reason => {
               return reject(reason);          }        )      })    })  }}

race测试代码（其他自行测试吧。。）

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