Promise then方法的参数变成可选参数

const PENDING = 'pending' // 等待
const FULFILLED = 'fulfilled' // 成功
const REJECTED = 'rejected'// 失败
class MyPromise {
  constructor(executor) {
    // 捕获执行器错误
    try{
      executor(this.resolve, this.reject)
    } catch (e) {
      this.reject(e)
    }
  }
  status = PENDING
  value = undefined
  reason = undefined
  successCallback = []
  failCallback = []
  resolve = value => {
    if (this.status !== PENDING) return
    // 将状态修改为成功
    this.status = FULFILLED
    // 保存成功之后的值
    this.value = value
    // this.successCallback && this.successCallback(this.value)
    while(this.successCallback.length) {
      this.successCallback.shift()()
    }
  }
  reject = reason => {
    if (this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
    // this.failCallback && this.failCallback(this.reason)
    while(this.failCallback.length) {
      this.failCallback.shift()()
    }
  }
  then(successCallback, failCallback) {
    successCallback = successCallback ? successCallback : value => value 
    failCallback = failCallback ? failCallback : reason => {throw reason}
    let promise2 = new MyPromise((resolve, reject) => {
      if (this.status === FULFILLED) {
        setTimeout(() => {
          try {
            let x = successCallback(this.value)
            // 判断x的值是普通值还是promise对象
            // 如果是普通值，直接调用resolve
            // 如果是promise对象，查看promise对象返回的结果
            // 再根据promise对象返回的结果，决定调用resolve还是reject
            resolvePromise(promise2, x, resolve, reject)
          } catch(e) {
            reject(e)
          }
        }, 0)
      } else if (this.status === REJECTED) {
        setTimeout(() => {
          try {
            let x = failCallback(this.reason)
            // 判断x的值是普通值还是promise对象
            // 如果是普通值，直接调用resolve
            // 如果是promise对象，查看promise对象返回的结果
            // 再根据promise对象返回的结果，决定调用resolve还是reject
            resolvePromise(promise2, x, resolve, reject)
          } catch(e) {
            reject(e)
          }
        }, 0)
      } else {
        // 等待
        // 讲成功回调和失败回调进行存储
        this.successCallback.push(() => {
          setTimeout(() => {
            try {
              let x = successCallback(this.value)
              // 判断x的值是普通值还是promise对象
              // 如果是普通值，直接调用resolve
              // 如果是promise对象，查看promise对象返回的结果
              // 再根据promise对象返回的结果，决定调用resolve还是reject
              resolvePromise(promise2, x, resolve, reject)
            } catch(e) {
              reject(e)
            }
          }, 0)
        })
        this.failCallback.push(() => {
          setTimeout(() => {
            try {
              let x = failCallback(this.reason)
              // 判断x的值是普通值还是promise对象
              // 如果是普通值，直接调用resolve
              // 如果是promise对象，查看promise对象返回的结果
              // 再根据promise对象返回的结果，决定调用resolve还是reject
              resolvePromise(promise2, x, resolve, reject)
            } catch(e) {
              reject(e)
            }
          }, 0)
        })
      }
    })
    return promise2
  }
}

function resolvePromise(promise2, x, resolve, reject) {
  if (promise2 === x) {
   return reject(new TypeError('Chaining cycle detected for promise #<Promise>'))
  }
  if (x instanceof MyPromise) {
    // x.then(value => resolve(value), reason => reject(reason))
    x.then(resolve, reject)
  } else {
    resolve(x)
  }
}

module.exports = MyPromise

let pro = new MyPromise((resolve, reject) => {
  resolve('hello')
  reject('失败')
})
pro.then().then().then(value => console.log(value), reason => console.log(reason))