promise4.js

// 解决Promise串行链式的延续问题，每次then都需要返回一个promise
class Promise {
  constructor (executor) {
    this.value = undefined
    this.status = 'pending'
    this.children = []
    executor(value => {
      this.status = 'resolve',
      this.setValue(value)
    }, reason => {
      this.status = 'rejected'
      this.setValue(reason)
    })
  }

  then(onResolved) {
    var child = new Promise(() => {})
    child.onResolved = onResolved
    this.children.push(child)
    return this
  }

  setValue (value) {
    this.value = value
    this.children.forEach(child => {
      var ret = child.onResolved(this.value)
      this.value = ret
    })
  }
}

var promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('promise1')
  },300)
})
promise.then(value1 => {
  console.log('value1',value1)
  value1 = 'promise2'
  return value1
}).then(value2 => {
  console.log('value2',value2)
})