手写Promise

1. 简易版Promise，只能调用一次then,不能链式调用:

class Promise {
  constructor(executor) {
    this.status = 'pending';
    this.value = undefined;
    this.reson = undefined;
    // 成功存放的数组
    this.onResolvedCallbacks = [];
    // 失败存放法数组
    this.onRejectedCallbacks = [];
    // 成功时调用
    let resolve = value => {
      setTimeout(() => {
        if (this.status === 'pending') {
          this.status = 'fulfilled';
          this.value = value;
          // 一旦resolve执行，调用成功数组的函数
          this.onResolvedCallbacks.forEach(fn => {
            fn();
          });
        }
      });
    };
    // 失败时调用
    let reject = reson => {
      setTimeout(() => {
        if (this.status === 'pending') {
          this.status = 'rejected';
          this.reson = reson;
          // 一旦reject执行，调用失败数组的函数
          this.onRejectedCallbacks.forEach(fn => {
            fn();
          });
        }
      });
    };

    try {
      executor(resolve, reject);
    } catch (error) {
      reject(error);
    }
  }
  then(onFulfilled, onRejected) {
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
    onRejected =
      typeof onRejected === 'function'
        ? onRejected
        : err => {
            throw err;
          };
    setTimeout(() => {
      if (this.status === 'fulfilled') {
        onFulfilled(this.value);
      }
    });
    setTimeout(() => {
      if (this.status === 'rejected') {
        onRejected(this.reson);
      }
    });
    if (this.status === 'pending') {
      this.onResolvedCallbacks.push(() => {
        setTimeout(() => {
          onFulfilled(this.value);
        });
      });
      this.onRejectedCallbacks.push(() => {
        setTimeout(() => {
          onRejected(this.reson);
        });
      });
    }
  }
}
let premise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('0.123');
  });
});
premise.then(res => {
  console.log(res); // 0.123
});