JS手写一个自定义Promise操作示例

class MyPromise {
 constructor(executor) {
  let _this = this
  this.state = 'pending' // 当前状态
  this.value = undefined // 存储成功的值
  this.reason = undefined // 存储失败的值
  // 利用发布订阅模式，让Promise支持异步
  this.onFulfilledFunc = [] // 存储成功的回调
  this.onRejectedFunc = [] // 存储失败的回调

  function resolve (value) {
   // Promise对象已经由pending状态改变为了成功态（resolved）或是失败态（rejected）就不能再次更改状态了。因此我们在更新状态时要判断，如果当前状态是pending（等待态）才可更新
   if (_this.state === 'pending') {
    _this.value = value
    //依次执行成功回调
    _this.onFulfilledFunc.forEach(fn => fn(value))
    _this.state = 'resolved'
   }
  }

  function reject (reason) {
   // Promise对象已经由pending状态改变为了成功态（resolved）或是失败态（rejected）就不能再次更改状态了。因此我们在更新状态时要判断，如果当前状态是pending（等待态）才可更新
   if (_this.state === 'pending') {
    _this.reason = reason
    //依次执行失败回调
    _this.onRejectedFunc.forEach(fn => fn(reason))
    _this.state = 'rejected'
   }
  }

  try {
   // 当实例化Promise时，构造函数中就要马上调用传入的executor函数执行
   executor(resolve, reject)
  } catch (error) {
   reject(error)
  }
 }
 _resolvePromise (promise2, x, resolve, reject) {
  // 如果返回了自己的Promise对象，状态永远为等待态（pending），再也无法成为resolved或是rejected，程序会死掉，因此首先要处理它
  if (promise2 === x) {
   reject(new TypeError('Promise存在循环引用'))
  }
  if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
   // x可能是一个promise
   try {
    let then = x.then
    if (typeof then === 'function') {
     then.call(x, (y) => {
      _resolvePromise(promise2, y, resolve, reject)
     })
    } else {
     resolve(x)
    }
   } catch (err) {
    reject(err)
   }
  } else {
   //否则是个普通值
   resolve(x)
  }
 }
 then (onFulfilled, onRejected) {
  let promise2
  onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : function (val) { return val }
  onRejected = typeof onRejected === 'function' ? onRejected : function (reason) { throw reason }

  if (this.state === 'resolved') {
   promise2 = new MyPromise((resolve, reject) => {
    setTimeout(() => {
     try {
      let x = onFulfilled(this.value)
      this._resolvePromise(promise2, x, resolve, reject)
     } catch (error) {
      reject(error)
     }
    }, 0);
   })
  }

  if (this.state === 'rejected') {
   promise2 = new MyPromise((resolve, reject) => {
    setTimeout(() => {
     try {
      let x = onRejected(this.reason)
      this._resolvePromise(promise2, x, resolve, reject)
     } catch (error) {
      reject(error)
     }
    }, 0);
   })
  }

  if (this.state === 'pending') {
   promise2 = new MyPromise((resolve, reject) => {
    this.onFulfilledFunc.push(() => {
     setTimeout(() => {
      try {
       let x = onFulfilled(this.value)
       this._resolvePromise(promise2, x, resolve, reject)
      } catch (error) {
       reject(error)
      }
     }, 0);
    })

    this.onRejectedFunc.push(() => {
     setTimeout(() => {
      try {
       let x = onRejected(this.reason)
       this._resolvePromise(promise2, x, resolve, reject)
      } catch (error) {
       reject(error)
      }
     }, 0);
    })
   })
  }

  return promise2
 }
}

var promise = new MyPromise((resolve, reject) => {
 console.log(1)
 setTimeout(() => {
  resolve(2)
 }, 1000);
 console.log(3)
}).then(value => console.log(value))