手写 Promise2

Promise 三个状态

1. pending
2. fulfilled
3. rejected

executor 同步执行器，并为执行器定义需要的 resolve、reject

利用 try…catch ，catch 调用 reject

利用发布订阅设计模式解决异步代码的执行

• this.onResolvedCallback = []; this.onRejectedCallback = [];
• 在 then 方法中订阅

• resolve，reject 发布

  class MyPromise {
  constructor(executor) {
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onResolvedCallback = [];
    this.onRejectedCallback = [];
    const resolve = (value) => {
      this.state = 'fulfilled';
      this.value = value;
      this.onResolvedCallback.forEach((fn) => fn());
    },
          reject = (reason) => {
            this.state = 'rejected';
            this.reason = reason;
            this.onRejectedCallback.forEach((fn) => fn());
          };
    try {
      executor(resolve, reject);
    } catch (reason) {
      reject(reason);
    }
  }
  then(onFulfilled, onRejected) {
    let x;
    return new MyPromise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        try {
          x = onFulfilled(this.value);
          resolve();
        } catch (e) {
          reject(e);
        }
      }
      if (this.state === 'rejected') {
        try {
          x = onRejected(this.reason);
          resolve(x);
        } catch (e) {
          reject(e);
        }
      }
      if (this.state === 'pending') {
        this.onResolvedCallback.push(() => {
          try {
            x = onFulfilled(this.value);
            resolve(x);
          } catch (e) {
            reject(e);
          }
        });
        this.onRejectedCallback.push(() => {
          try {
            x = onRejected(this.reason);
            resolve(x);
          } catch (e) {
            reject(e);
          }
        });
      }
    });
  }
  }