自己的Promise

废话不多说，直接上代码：

class myPromise {
  constructor(fn) {
    this.status = 'pending';
    this.resolveCbs = [];
    this.rejectCbs = [];
    this.value = null;
    fn(this.resolve.bind(this), this.reject.bind(this))
  }
  resolve(val) {
    this.value = val;
    this.status = 'resolved';
    this.resolveCbs.forEach(cb => {
      cb(this.value);
    })
  }
  reject(err) {
    this.value = err;
    this.status = 'rejected';
    this.rejectCbs.forEach(cb => {
      cb(this.value);
    })
  }
  then(resolveCb, rejectCb) {
    if (this.status === 'resolved') {
      resolveCb(this.value);
    } else if (this.status === 'rejected') {
      rejectCb(this.value);
    } else {
      if (typeof resolveCb === 'function') {
        this.resolveCbs.push(resolveCb)
      }
      if (typeof resolveCb === 'function') {
        this.rejectCbs.push(resolveCb)
      }
    }
  }
}
new myPromise((resolve, reject) => {
  setTimeout(() => {
    resolve(456)
  }, 1000);
}).then(res => {
  console.log(res)
})

这个简版的Promise只能实现一层then，不能做链式。

来源：https://www.cnblogs.com/amiezhang/p/8006370.html