前端面试题 – 102. 实现一个简单的Promise类，包括then和catch方法。

内置属性status的流转：初始化（resolve、reject）、then、catch。并记录value和reason

class MyPromise {
  constructor(executor) {
    this.value = undefined;
    this.reason = undefined;
    this.status = 'pending';

    const resolve = (value) => {
      if (this.status === 'pending') {
        this.value = value;
        this.status = 'resolved';
      }
    };

    const reject = (reason) => {
      if (this.status === 'pending') {
        this.reason = reason;
        this.status = 'rejected';
      }
    };

    try {
      executor(resolve, reject);
    } catch (error) {
      reject(error);
    }
  }

  then(onFulfilled, onRejected) {
    if (this.status === 'resolved') {
      onFulfilled(this.value);
    } else if (this.status === 'rejected') {
      onRejected(this.reason);
    }
  }

  catch(onRejected) {
    if (this.status === 'rejected') {
      onRejected(this.reason);
    }
  }
}