JavaScript Promise

my-promise.js

class MyPromise {
  callbacks = []
  state = 'pending'
  value = null
  constructor(fn) {
    fn(this._resolve.bind(this))
  }
  then(onFulfilled) {
    return new Promise(resolve => {
      this._handle({
        onFulfilled: onFulfilled || null,
        resolve: resolve
      })
    })
  }
  _handle(callback) {
      if (this.state === 'pending') {
        this.callbacks.push(callback)
        return;
      }
      // 如果then没有传递任何东西
      if (!callback.onFulfilled) {
        callback.resolve(this.value);
        return;
      }
      var ret = callback.onFulfilled(this.value);
      callback.resolve(ret);
  }
  _resolve(value) {
    this.state = 'fulfilled';
    this.value = value
    this.callbacks.forEach(callback => this._handle(callback))
  }
}

// 测试
const mockAjax = (url, s, callback) => {
  setTimeout(() => {
    callback(url + '异步请求耗时' + s + '秒')
  }, s * 1000)
}

new MyPromise(resolve => {
  mockAjax('getUserId', 1, function (result) {
    resolve(result)
  })
}).then(result => {
  console.log(result);
  return new Promise(resolve => {
    mockAjax('getAgentId', 2, function (result) {
      resolve(result)
    })
  })
}).then(result => {
  console.log(result);
})