手写Promise

       // excutor：可以理解为传入一个函数为执行器
       function myPromise(excutor) {
          // 1. 执行结构
          let self = this
          self.status = 'pending' // 状态
          self.value = null //成功的值
          self.reason = null //失败原因

          // 8. 添加缓存数组
          self.onFulfilledCallbacks = []
          self.onRejectedCallbacks = []

          // 成功 4.判断状态做相应处理
          function resolve(value) {
              if (self.status === 'pending') {
                  self.value = value //保存成功结果
                  self.status = 'fulfilled'
                  // 10. 状态改变，依次取出
                  self.onFulfilledCallbacks.forEach(item => item(value))
              }
          }
          // 失败
          function reject(reason) {
              if (self.status === 'pending') {
                  self.reason = reason //保存失败原因
                  self.status = 'rejected'
                  // 10. 状态改变，依次取出
                  self.onRejectedCallbacks.forEach(item => item(reason))
              }
          }
          // 3.执行一遍
          try {
              excutor(resolve, reject)
          } catch (error) {
              reject(error)
          }
      }
      //2. then 关联到原型链上
      myPromise.prototype.then = function (onFulfilled, onRejected) {
          // 5.状态改变，调用then方法
          onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : function (data) { resolve(data) }
          onRejected = typeof onRejected === 'function' ? onRejected : function (err) { throw err }
          // 9. 先添加进去
          let self = this
          if (self.status === 'pending') {
              self.onFulfilledCallbacks.push(onFulfilled)
              self.onRejectedCallbacks.push(onRejected)
          }
      }

      let demo = new myPromise((resolve, reject) => {
          console.log("打印出内容")
          //6. 尝试异步
          setTimeout(() => {
              resolve('异步成功')
          }, 500)
      })

      // 执行 7.异步失败
      demo.then(data => console.log(data)) //异步成功