深度剖析：手写一个Promise源码

时间 2020-11-16 标签编程 segmentfault 数组 promise 并发异步异步编程函数 this

1、Promise核心逻辑实现
2、在 Promise 类中加入异步逻辑
3、实现 then 方法屡次调用添加多个处理函数
4、实现then方法的链式调用
5、then方法链式调用识别 Promise 对象自返回
6、捕获错误及 then 链式调用其余状态代码补充编程
- 1. 捕获执行器的错误
- 1. then执行的时候报错捕获
- 1. 错误以后的链式调用
- 1. 异步状态下链式调用
7、将then方法的参数变成可选参数
8、promise.all方法的实现
9、Promise.resolve方法的实现
10、finally 方法的实现
11、catch方法的实现
Promise所有代码整合

这里只是对Promise源码的实现作一个剖析，若是想对Promise总体有个了解， 
请看 ES6（十一）—— Promise（更优的异步编程解决方案）

1、Promise核心逻辑实现

首先分析其原理segmentfault

promise就是一个类 

在执行类的时候须要传递一个执行器进去，执行器会当即执行数组

Promise中有三种状态，分别为成功-fulfilled 失败-rejected 等待-pending 
pending -> fulfilled 
pending -> rejected 
一旦状态肯定就不可更改

resolve 和 reject函数是用来更改状态的 

resolve：fulfilled 
reject：rejectedpromise

then方法内部作的事情就是判断状态

若是状态是成功，调用成功回调函数 
若是状态是失败，就调用失败回调函数 
then方法是被定义在原型对象中的并发

then成功回调有一个参数，表示成功以后的值；then失败回调有一个参数，表示失败后的缘由

<PS:本文myPromise.js是源码文件，promise.js是使用promise文件>异步

// myPromise.js
// 定义成常量是为了复用且代码有提示
const PENDING = 'pending' // 等待
const FULFILLED = 'fulfilled' // 成功
const REJECTED = 'rejected' // 失败
// 定义一个构造函数
class MyPromise {
  constructor (exector) {
    // exector是一个执行器，进入会当即执行，并传入resolve和reject方法
    exector(this.resolve, this.reject)
  }

  // 实例对象的一个属性，初始为等待
  status = PENDING
  // 成功以后的值
  value = undefined
  // 失败以后的缘由
  reason = undefined

  // resolve和reject为何要用箭头函数？
  // 若是直接调用的话，普通函数this指向的是window或者undefined
  // 用箭头函数就可让this指向当前实例对象
  resolve = value => {
    // 判断状态是否是等待，阻止程序向下执行
    if(this.status !== PENDING) return
    // 将状态改为成功
    this.status = FULFILLED
    // 保存成功以后的值
    this.value = value
  }

  reject = reason => {
    if(this.status !== PENDING) return
    // 将状态改成失败
    this.status = REJECTED
    // 保存失败以后的缘由
    this.reason = reason
  }

  then (successCallback, failCallback) {
    //判断状态
    if(this.status === FULFILLED) {
      // 调用成功回调，而且把值返回
      successCallback(this.value)
    } else if (this.status === REJECTED) {
      // 调用失败回调，而且把缘由返回
      failCallback(this.reason)
    }
  }
}

module.exports = MyPromise

//promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
   resolve('success')
   reject('err')
 })

 promise.then(value => {
   console.log('resolve', value)
 }, reason => {
   console.log('reject', reason)
 })

2、在 Promise 类中加入异步逻辑

上面是没有通过异步处理的，若是有异步逻辑加进来，会有一些问题异步编程

//promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
  // 主线程代码当即执行，setTimeout是异步代码，then会立刻执行，
  // 这个时候判断promise状态，状态是pending，然而以前并无判断等待这个状态
  setTimeout(() => {
    resolve('success')
  }, 2000); 
 })

 promise.then(value => {
   console.log('resolve', value)
 }, reason => {
   console.log('reject', reason)
 })

下面修改这个代码函数

// myPromise.js
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
class MyPromise {
  constructor (exector) {
    exector(this.resolve, this.reject)
  }


  status = PENDING
  value = undefined
  reason = undefined
  // 定义一个成功回调参数
  successCallback = undefined
  // 定义一个失败回调参数
  failCallback = undefined

  resolve = value => {
    if(this.status !== PENDING) return
    this.status = FULFILLED
    this.value = value
    // 判断成功回调是否存在，若是存在就调用
    this.successCallback && this.successCallback(this.value)
  }

  reject = reason => {
    if(this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
    // 判断失败回调是否存在，若是存在就调用
    this.failCallback && this.failCallback(this.reason)
  }

  then (successCallback, failCallback) {
    if(this.status === FULFILLED) {
      successCallback(this.value)
    } else if (this.status === REJECTED) {
      failCallback(this.reason)
    } else {
      // 等待
      // 由于并不知道状态，因此将成功回调和失败回调存储起来
      // 等到执行成功失败函数的时候再传递
      this.successCallback = successCallback
      this.failCallback = failCallback
    }
  }
}

module.exports = MyPromise

3、实现 then 方法屡次调用添加多个处理函数

promise的then方法是能够被屡次调用的。ui

这里若是有三个then的调用，this

若是是同步回调，那么直接返回当前的值就行；
若是是异步回调，那么保存的成功失败的回调，须要用不一样的值保存，由于都互不相同。

以前的代码须要改进。

//promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
  setTimeout(() => {
    resolve('success')
  }, 2000); 
 })

 promise.then(value => {
   console.log(1)
   console.log('resolve', value)
 })
 
 promise.then(value => {
  console.log(2)
  console.log('resolve', value)
})

promise.then(value => {
  console.log(3)
  console.log('resolve', value)
})

保存到数组中，最后统一执行

// myPromise.js
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
class MyPromise {
  constructor (exector) {
    exector(this.resolve, this.reject)
  }


  status = PENDING
  value = undefined
  reason = undefined
  // 定义一个成功回调参数，初始化一个空数组
  successCallback = []
  // 定义一个失败回调参数，初始化一个空数组
  failCallback = []

  resolve = value => {
    if(this.status !== PENDING) return
    this.status = FULFILLED
    this.value = value
    // 判断成功回调是否存在，若是存在就调用
    // 循环回调数组. 把数组前面的方法弹出来而且直接调用
    // shift方法是在数组中删除值，每执行一个就删除一个，最终变为0
    while(this.successCallback.length) this.successCallback.shift()(this.value)
  }

  reject = reason => {
    if(this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
    // 判断失败回调是否存在，若是存在就调用
    // 循环回调数组. 把数组前面的方法弹出来而且直接调用
    while(this.failCallback.length) this.failCallback.shift()(this.reason)
  }

  then (successCallback, failCallback) {
    if(this.status === FULFILLED) {
      successCallback(this.value)
    } else if (this.status === REJECTED) {
      failCallback(this.reason)
    } else {
      // 等待
      // 将成功回调和失败回调都保存在数组中
      this.successCallback.push(successCallback)
      this.failCallback.push(failCallback)
    }
  }
}

module.exports = MyPromise

4、实现then方法的链式调用

then方法要链式调用那么就须要返回一个promise对象，
then方法的return返回值做为下一个then方法的参数
then方法还一个return一个promise对象，那么若是是一个promise对象，那么就须要判断它的状态

// promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
// 目前这里只处理同步的问题
    resolve('success')
})

function other () {
   return new MyPromise((resolve, reject) =>{
        resolve('other')
   })
}
promise.then(value => {
   console.log(1)
   console.log('resolve', value)
   return other()
}).then(value => {
  console.log(2)
  console.log('resolve', value)
})

// myPromise.js
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
class MyPromise {
  constructor (exector) {
    exector(this.resolve, this.reject)
  }

  status = PENDING
  value = undefined
  reason = undefined
  successCallback = []
  failCallback = []

  resolve = value => {
    if(this.status !== PENDING) return
    this.status = FULFILLED
    this.value = value
    while(this.successCallback.length) this.successCallback.shift()(this.value)
  }

  reject = reason => {
    if(this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
    while(this.failCallback.length) this.failCallback.shift()(this.reason)
  }

  then (successCallback, failCallback) {
    // then方法返回第一个promise对象
    let promise2 = new Promise((resolve, reject) => {
      if(this.status === FULFILLED) {
        // x是上一个promise回调函数的return返回值
        // 判断 x 的值时普通值仍是promise对象
        // 若是是普通纸 直接调用resolve
        // 若是是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仍是reject
        let x = successCallback(this.value)
        resolvePromise(x, resolve, reject)
      } else if (this.status === REJECTED) {
        failCallback(this.reason)
      } else {
        this.successCallback.push(successCallback)
        this.failCallback.push(failCallback)
      }
    });
    return promise2
  }
}

function resolvePromise(x, resolve, reject) {
  // 判断x是否是其实例对象
  if(x instanceof MyPromise) {
    // promise 对象
    // x.then(value => resolve(value), reason => reject(reason))
    // 简化以后
    x.then(resolve, reject)
  } else{
    // 普通值
    resolve(x)
  }
}

module.exports = MyPromise

5、then方法链式调用识别 Promise 对象自返回

若是then方法返回的是本身的promise对象，则会发生promise的嵌套，这个时候程序会报错

var promise = new Promise((resolve, reject) => {
  resolve(100)
})
var p1 = promise.then(value => {
  console.log(value)
  return p1
})

// 100
// Uncaught (in promise) TypeError: Chaining cycle detected for promise #<Promise>

因此为了不这种状况，咱们须要改造一下then方法

// myPromise.js
const { rejects } = require("assert")

const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
class MyPromise {
  constructor (exector) {
    exector(this.resolve, this.reject)
  }


  status = PENDING
  value = undefined
  reason = undefined
  successCallback = []
  failCallback = []

  resolve = value => {
    if(this.status !== PENDING) return
    this.status = FULFILLED
    this.value = value
    while(this.successCallback.length) this.successCallback.shift()(this.value)
  }

  reject = reason => {
    if(this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
    while(this.failCallback.length) this.failCallback.shift()(this.reason)
  }

  then (successCallback, failCallback) {
    let promise2 = new Promise((resolve, reject) => {
      if(this.status === FULFILLED) {
        // 由于new Promise须要执行完成以后才有promise2，同步代码中没有pormise2，
        // 因此这部分代码须要异步执行
        setTimeout(() => {
          let x = successCallback(this.value)
          //须要判断then以后return的promise对象和原来的是否是同样的，
          //判断x和promise2是否相等，因此给resolvePromise中传递promise2过去
          resolvePromise(promise2, x, resolve, reject)
        }, 0);
      } else if (this.status === REJECTED) {
        failCallback(this.reason)
      } else {
        this.successCallback.push(successCallback)
        this.failCallback.push(failCallback)
      }
    });
    return promise2
  }
}

function resolvePromise(promise2, x, resolve, reject) {
  // 若是相等了，说明return的是本身，抛出类型错误并返回
  if (promise2 === x) {
    return reject(new TypeError('Chaining cycle detected for promise #<Promise>'))
  }
  if(x instanceof MyPromise) {
    x.then(resolve, reject)
  } else{
    resolve(x)
  }
}

module.exports = MyPromise

// promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
    resolve('success')
})
 
// 这个时候将promise定义一个p1，而后返回的时候返回p1这个promise
let p1 = promise.then(value => {
   console.log(1)
   console.log('resolve', value)
   return p1
})
 
// 运行的时候会走reject
p1.then(value => {
  console.log(2)
  console.log('resolve', value)
}, reason => {
  console.log(3)
  console.log(reason.message)
})

// 1
// resolve success
// 3
// Chaining cycle detected for promise #<Promise>

6、捕获错误及 then 链式调用其余状态代码补充

目前咱们在Promise类中没有进行任何处理，因此咱们须要捕获和处理错误。

1. 捕获执行器的错误

捕获执行器中的代码，若是执行器中有代码错误，那么promise的状态要弄成错误状态

// myPromise.js
constructor (exector) {
    // 捕获错误，若是有错误就执行reject
    try {
        exector(this.resolve, this.reject)
    } catch (e) {
        this.reject(e)
    }
}

// promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
    // resolve('success')
    throw new Error('执行器错误')
})
 
promise.then(value => {
  console.log(1)
  console.log('resolve', value)
}, reason => {
  console.log(2)
  console.log(reason.message)
})

//2
//执行器错误

2. then执行的时候报错捕获

// myPromise.js
then (successCallback, failCallback) {
    let promise2 = new Promise((resolve, reject) => {
        if(this.status === FULFILLED) {
            setTimeout(() => {
                // 若是回调中报错的话就执行reject
                try {
                    let x = successCallback(this.value)
                    resolvePromise(promise2, x, resolve, reject)
                } catch (e) {
                    reject(e)
                }
            }, 0);
        } else if (this.status === REJECTED) {
            failCallback(this.reason)
        } else {
            this.successCallback.push(successCallback)
            this.failCallback.push(failCallback)
        }
    });
    return promise2
}

// promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
    resolve('success')
    // throw new Error('执行器错误')
 })
 
// 第一个then方法中的错误要在第二个then方法中捕获到
promise.then(value => {
  console.log(1)
  console.log('resolve', value)
  throw new Error('then error')
}, reason => {
  console.log(2)
  console.log(reason.message)
}).then(value => {
  console.log(3)
  console.log(value);
}, reason => {
  console.log(4)
  console.log(reason.message)
})

// 1
// resolve success
// 4
// then error

3. 错误以后的链式调用

// myPromise.js
then (successCallback, failCallback) {
    let promise2 = new Promise((resolve, reject) => {
        if(this.status === FULFILLED) {
            setTimeout(() => {
                try {
                    let x = successCallback(this.value)
                    resolvePromise(promise2, x, resolve, reject)
                } catch (e) {
                    reject(e)
                }
            }, 0)
        // 在状态是reject的时候对返回的promise进行处理
        } else if (this.status === REJECTED) {
            setTimeout(() => {
                // 若是回调中报错的话就执行reject
                try {
                    let x = failCallback(this.reason)
                    resolvePromise(promise2, x, resolve, reject)
                } catch (e) {
                    reject(e)
                }
            }, 0)
        } else {
            this.successCallback.push(successCallback)
            this.failCallback.push(failCallback)
        }
    });
    return promise2
}

//promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
    // resolve('success')
    throw new Error('执行器错误')
 })
 
 // 第一个then方法中的错误要在第二个then方法中捕获到
 promise.then(value => {
  console.log(1)
  console.log('resolve', value)
}, reason => {
  console.log(2)
  console.log(reason.message)
  return 100
}).then(value => {
  console.log(3)
  console.log(value);
}, reason => {
  console.log(4)
  console.log(reason.message)
})

// 2
// 执行器错误
// 3
// 100

4. 异步状态下链式调用

仍是要处理一下若是promise里面有异步的时候，then的链式调用的问题。

// myPromise.js
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
class MyPromise {
  constructor (exector) {
    // 捕获错误，若是有错误就执行reject
    try {
      exector(this.resolve, this.reject)
    } catch (e) {
      this.reject(e)
    }
  }


  status = PENDING
  value = undefined
  reason = undefined
  successCallback = []
  failCallback = []

  resolve = value => {
    if(this.status !== PENDING) return
    this.status = FULFILLED
    this.value = value
    // 异步回调传值
    // 调用的时候不须要传值，由于下面push到里面的时候已经处理好了
    while(this.successCallback.length) this.successCallback.shift()()
  }

  reject = reason => {
    if(this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
    // 异步回调传值
    // 调用的时候不须要传值，由于下面push到里面的时候已经处理好了
    while(this.failCallback.length) this.failCallback.shift()()
  }

  then (successCallback, failCallback) {
    let promise2 = new Promise((resolve, reject) => {
      if(this.status === FULFILLED) {
        setTimeout(() => {
          // 若是回调中报错的话就执行reject
          try {
            let x = successCallback(this.value)
            resolvePromise(promise2, x, resolve, reject)
          } catch (e) {
            reject(e)
          }
        }, 0)
      } else if (this.status === REJECTED) {
        setTimeout(() => {
          // 若是回调中报错的话就执行reject
          try {
            let x = failCallback(this.reason)
            resolvePromise(promise2, x, resolve, reject)
          } catch (e) {
            reject(e)
          }
        }, 0)
      } else {
        // 处理异步的成功错误状况
        this.successCallback.push(() => {
          setTimeout(() => {
            // 若是回调中报错的话就执行reject
            try {
              let x = successCallback(this.value)
              resolvePromise(promise2, x, resolve, reject)
            } catch (e) {
              reject(e)
            }
          }, 0)
        })
        this.failCallback.push(() => {
          setTimeout(() => {
            // 若是回调中报错的话就执行reject
            try {
              let x = failCallback(this.reason)
              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(resolve, reject)
  } else{
    resolve(x)
  }
}

module.exports = MyPromise

// promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
  // 一个异步方法
  setTimeout(() =>{
    resolve('succ')
  },2000)
})
 
 promise.then(value => {
  console.log(1)
  console.log('resolve', value)
  return 'aaa'
}, reason => {
  console.log(2)
  console.log(reason.message)
  return 100
}).then(value => {
  console.log(3)
  console.log(value);
}, reason => {
  console.log(4)
  console.log(reason.message)
})

// 1
// resolve succ
// 3
// aaa

7、将then方法的参数变成可选参数

then方法的两个参数都是可选参数，咱们能够不传参数。
下面的参数能够传递到最后进行返回

var promise = new Promise((resolve, reject) => {
      resolve(100)
    })
    promise
      .then()
      .then()
      .then()
      .then(value => console.log(value))
// 在控制台最后一个then中输出了100

// 这个至关于
promise
  .then(value => value)
  .then(value => value)
  .then(value => value)
  .then(value => console.log(value))

因此咱们修改一下then方法

// myPromise.js
then (successCallback, failCallback) {
    // 这里进行判断，若是有回调就选择回调，若是没有回调就传一个函数，把参数传递
    successCallback = successCallback ? successCallback : value => value
    // 错误函数也是进行赋值，把错误信息抛出
    failCallback = failCallback ? failCallback : reason => {throw reason}
    let promise2 = new Promise((resolve, reject) => {
        ...
    })
    ...
}


// 简化也能够这样写
then (successCallback = value => value, failCallback = reason => {throw reason}) {
    ···
}

resolve以后

// promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
  resolve('succ')
})
 
promise.then().then().then(value => console.log(value))

// succ

reject以后

// promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
  reject('err')
})
 
 promise.then().then().then(value => console.log(value), reason => console.log(reason))

// err

8、promise.all方法的实现

promise.all方法是解决异步并发问题的

// 若是p1是两秒以后执行的，p2是当即执行的，那么根据正常的是p2在p1的前面。
// 若是咱们在all中指定了执行顺序，那么会根据咱们传递的顺序进行执行。
function p1 () {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('p1')
    }, 2000)
  })
}

function p2 () {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('p2')
    },0)
  })
}

Promise.all(['a', 'b', p1(), p2(), 'c']).then(result => {
  console.log(result)
  // ["a", "b", "p1", "p2", "c"]
})

分析一下：

all方法接收一个数组，数组中能够是普通值也能够是promise对象
数组中值得顺序必定是咱们获得的结果的顺序
promise返回值也是一个promise对象，能够调用then方法
若是数组中全部值是成功的，那么then里面就是成功回调，若是有一个值是失败的，那么then里面就是失败的
使用all方法是用类直接调用，那么all必定是一个静态方法

//myPromise.js
static all (array) {
    //  结果数组
    let result = []
    // 计数器
    let index = 0
    return new Promise((resolve, reject) => {
      let addData = (key, value) => {
        result[key] = value
        index ++
        // 若是计数器和数组长度相同，那说明全部的元素都执行完毕了，就能够输出了
        if(index === array.length) {
          resolve(result)
        }
      }
      // 对传递的数组进行遍历
      for (let i = 0; i < array.lengt; i++) {
        let current = array[i]
        if (current instanceof MyPromise) {
          // promise对象就执行then，若是是resolve就把值添加到数组中去，若是是错误就执行reject返回
          current.then(value => addData(i, value), reason => reject(reason))
        } else {
          // 普通值就加到对应的数组中去
          addData(i, array[i])
        }
      }
    })
}

// promise.js
const MyPromise = require('./myPromise')
function p1 () {
  return new MyPromise((resolve, reject) => {
    setTimeout(() => {
      resolve('p1')
    }, 2000)
  })
}

function p2 () {
  return new MyPromise((resolve, reject) => {
    setTimeout(() => {
      resolve('p2')
    },0)
  })
}

Promise.all(['a', 'b', p1(), p2(), 'c']).then(result => {
  console.log(result)
  // ["a", "b", "p1", "p2", "c"]
})

9、Promise.resolve方法的实现

若是参数就是一个promise对象，直接返回，若是是一个值，那么须要生成一个promise对象，把值进行返回
是Promise类的一个静态方法

// myPromise.js
static resolve (value) {
    // 若是是promise对象，就直接返回
    if(value instanceof MyPromise) return value
    // 若是是值就返回一个promise对象，并返回值
    return new MyPromise(resolve => resolve(value))
}

// promise.js
const MyPromise = require('./myPromise')
function p1 () {
  return new MyPromise((resolve, reject) => {
    setTimeout(() => {
      resolve('p1')
    }, 2000)
  })
}


Promise.resolve(100).then(value => console.log(value))
Promise.resolve(p1()).then(value => console.log(value))
// 100
// 2s 以后输出 p1

10、finally 方法的实现

不管当前最终状态是成功仍是失败，finally都会执行
咱们能够在finally方法以后调用then方法拿到结果
这个函数是在原型对象上用的

// myPromise.js
finally (callback) {
    // 如何拿到当前的promise的状态，使用then方法，并且无论怎样都返回callback
    // 并且then方法就是返回一个promise对象，那么咱们直接返回then方法调用以后的结果便可
    // 咱们须要在回调以后拿到成功的回调，因此须要把value也return
    // 失败的回调也抛出缘由
    // 若是callback是一个异步的promise对象，咱们还须要等待其执行完毕，因此须要用到静态方法resolve
    return this.then(value => {
      // 把callback调用以后返回的promise传递过去，而且执行promise，且在成功以后返回value
      return MyPromise.resolve(callback()).then(() => value)
    }, reason => {
      // 失败以后调用的then方法，而后把失败的缘由返回出去。
      return MyPromise.resolve(callback()).then(() => { throw reason })
    })
}

// promise.js
const MyPromise = require('./myPromise')
function p1 () {
  return new MyPromise((resolve, reject) => {
    setTimeout(() => {
      resolve('p1')
    }, 2000)
  })
}

function p2 () {
  return new MyPromise((resolve, reject) => {
    reject('p2 reject')
  })
}

p2().finally(() => {
  console.log('finallyp2')
  return p1()
}).then(value => {
  console.log(value)
}, reason => {
  console.log(reason)
})

// finallyp2
// 两秒以后执行p2 reject

11、catch方法的实现

catch方法是为了捕获promise对象的全部错误回调的
直接调用then方法，而后成功的地方传递undefined，错误的地方传递reason
catch方法是做用在原型对象上的方法

// myPromise.js
catch (failCallback) {
    return this.then(undefined, failCallback)
}

// promise.js
const MyPromise = require('./myPromise')

function p2 () {
  return new MyPromise((resolve, reject) => {
    reject('p2 reject')
  })
}

p2()
  .then(value => {
    console.log(value)
  })
  .catch(reason => console.log(reason))

// p2 reject

Promise所有代码整合

// myPromise.js
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
class MyPromise {
  constructor (exector) {
    try {
      exector(this.resolve, this.reject)
    } catch (e) {
      this.reject(e)
    }
  }


  status = PENDING
  value = undefined
  reason = undefined
  successCallback = []
  failCallback = []

  resolve = value => {
    if(this.status !== PENDING) return
    this.status = FULFILLED
    this.value = value
    while(this.successCallback.length) this.successCallback.shift()()
  }

  reject = reason => {
    if(this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
    while(this.failCallback.length) this.failCallback.shift()()
  }

  then (successCallback = value => value, failCallback = reason => {throw reason}) {
    let promise2 = new Promise((resolve, reject) => {
      if(this.status === FULFILLED) {
        setTimeout(() => {
          try {
            let x = successCallback(this.value)
            resolvePromise(promise2, x, resolve, reject)
          } catch (e) {
            reject(e)
          }
        }, 0)
      } else if (this.status === REJECTED) {
        setTimeout(() => {
          try {
            let x = failCallback(this.reason)
            resolvePromise(promise2, x, resolve, reject)
          } catch (e) {
            reject(e)
          }
        }, 0)
      } else {
        this.successCallback.push(() => {
          setTimeout(() => {
            try {
              let x = successCallback(this.value)
              resolvePromise(promise2, x, resolve, reject)
            } catch (e) {
              reject(e)
            }
          }, 0)
        })
        this.failCallback.push(() => {
          setTimeout(() => {
            try {
              let x = failCallback(this.reason)
              resolvePromise(promise2, x, resolve, reject)
            } catch (e) {
              reject(e)
            }
          }, 0)
        })
      }
    });
    return promise2
  }

  finally (callback) {
    // 如何拿到当前的promise的状态，使用then方法，并且无论怎样都返回callback
    // 并且then方法就是返回一个promise对象，那么咱们直接返回then方法调用以后的结果便可
    // 咱们须要在回调以后拿到成功的回调，因此须要把value也return
    // 失败的回调也抛出缘由
    // 若是callback是一个异步的promise对象，咱们还须要等待其执行完毕，因此须要用到静态方法resolve
    return this.then(value => {
      // 把callback调用以后返回的promise传递过去，而且执行promise，且在成功以后返回value
      return MyPromise.resolve(callback()).then(() => value)
    }, reason => {
      // 失败以后调用的then方法，而后把失败的缘由返回出去。
      return MyPromise.resolve(callback()).then(() => { throw reason })
    })
  }

  catch (failCallback) {
    return this.then(undefined, failCallback)
  }

  static all (array) {
    let result = []
    let index = 0
    return new Promise((resolve, reject) => {
      let addData = (key, value) => {
        result[key] = value
        index ++
        if(index === array.length) {
          resolve(result)
        }
      }
      for (let i = 0; i < array.lengt; i++) {
        let current = array[i]
        if (current instanceof MyPromise) {
          current.then(value => addData(i, value), reason => reject(reason))
        } else {
          addData(i, array[i])
        }
      }
    })
  }

  static resolve (value) {
    if(value instanceof MyPromise) return value
    return new MyPromise(resolve => resolve(value))
  }
}

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(resolve, reject)
  } else{
    resolve(x)
  }
}

module.exports = MyPromise