面试常考题之一： 手写一个Promise
本文简单实现了一个Promise

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function myPromise (executor) {
  var _this = this
  this.state = 'pending'
  this.value = undefined
  this.reason = undefined
  this.onFulfilledFunc = []
  this.onRejectedFunc = []

  executor(resolve, reject)
  function resolve (value) {
    if (_this.state === 'pending') {
      _this.value = value
      value && _this.onFulfilledFunc.forEach(fn => fn(value))
      _this.state = 'resolved'
    }
  }
  function reject (reason) {
    if (_this.state === 'pending') {
      _this.reason = reason
      _this.onRejectedFunc.forEach(fn => fn(reason))
      _this.state = 'rejected'
    }
  }
}

myPromise.prototype.then = function (onFulfilled, onRejected) {
  const _this = this
  if (_this.state === 'pending') {
    typeof onFulfilled === 'function' && _this.onFulfilledFunc.push(() => onFulfilled(this.value))
    typeof onRejected === 'function' && _this.onRejectedFunc.push(() => onRejected(this.reason))
  }

  return new myPromise((resolve, reject) => {
    typeof onFulfilled === 'function' && resolve(onFulfilled(this.value))
    typeof onRejected === 'function' && reject(onRejected(this.reason))
  })
}

var p = new myPromise((resolve, reject) => {
  setTimeout(() => {
    resolve('aaa')
  }, 500)
})

p.then(data1 => {console.log(data1);return 'bbb'})
  .then(data2 => console.log(data2))