1. //resolve和reject
    2. function Promise(fn) {
    3.   this.status = 'pending'
    4.   this.value = ''
    5.   const _this = this
    6.   try {
    7.     fn(resolve, reject)
    8.   } catch (e) {
    9.     reject(e)
    10.   }
    11.   function resolve(value) {
    12.     _this.value = value
    13.     _this.status = 'resloved'
    14.   }
    15.   function reject(value) {
    16.     _this.value = value
    17.     _this.status = 'rejected'
    18.   }
    19. }
    20. //then方法实现
    21. Promise.prototype.then = function (onReslved, onRejected) {
    22.   if (this.status === 'resolved') {
    23.     onReslved(this.value)
    24.   }
    25.   if (self.status === 'rejected') {
    26.     onRejected(this.reason);
    27.   }
    28. }