class Promise {

  // @param {function} eg: (resolve, reject) => { resolve() }
  constructor (exector) {
    this._queue = []  // 存储通过 then 方法传入的的函数
    this._exector = exector  // 调用 then 方法时执行该函数
  }

  then (onResolved, onRejected) {
    this._queue.push([onResolved, onRejected])
    this._exector(this._resolve.bind(this), this._reject.bind(this))
  }

  _resolve (data) {
    this._next(0, data)
  }

  _reject (reason) {
    this._next(1, reason)
  }

  // state 0 代表 resolve, 1 代表 reject
  _next (state, val) {
    let arr = this._queue.shift()
    if (arr) {
      arr[state](val)
    }
  }

}