SOURCE

const PENDING = 'pending'
const RESOLVED = 'resolved'
const REJECTED = 'rejected'

function MyPromise(fn) {
  const that = this
  that.state = PENDING
  that.value = null
  that.resolvedCallbacks = []
  that.rejectedCallbacks = []
  // 待完善 resolve 和 reject 函数
  function resolve(value) {
  if (that.state === PENDING) {
    that.state = RESOLVED
    that.value = value
    that.resolvedCallbacks.map(cb => cb(that.value))
  }
}

    function reject(value) {
      if (that.state === PENDING) {
        that.state = REJECTED
        that.value = value
        that.rejectedCallbacks.map(cb => cb(that.value))
      }
    }
      // 待完善执行 fn 函数
      try {
          fn(resolve, reject)
        } catch (e) {
          reject(e)
        }
}

MyPromise.prototype.then = function(onFulfilled, onRejected) {
  const that = this
  onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v
  onRejected =
    typeof onRejected === 'function'
      ? onRejected
      : r => {
          throw r
        }
  if (that.state === PENDING) {
    that.resolvedCallbacks.push(onFulfilled)
    that.rejectedCallbacks.push(onRejected)
  }
  if (that.state === RESOLVED) {
    onFulfilled(that.value)
  }
  if (that.state === REJECTED) {
    onRejected(that.value)
  }
}

function log(v){
    console.log(v)
}

let p = new MyPromise(function(resolve, reject){
    setTimeout(()=>{
        resolve('1')
    },2000)   
}).then(log)