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关于javascript:深度剖析手写一个Promise源码

作者：

jiezi

在

目录

  • 一、Promise外围逻辑实现
  • 二、在 Promise 类中退出异步逻辑
  • 三、实现 then 办法屡次调用增加多个处理函数
  • 四、实现then办法的链式调用
  • 五、then办法链式调用辨认 Promise 对象自返回

  • 六、捕捉谬误及 then 链式调用其余状态代码补充

      1. 捕捉执行器的谬误
      1. then执行的时候报错捕捉
      1. 谬误之后的链式调用
      1. 异步状态下链式调用
  • 七、将then办法的参数变成可选参数
  • 八、promise.all办法的实现
  • 九、Promise.resolve办法的实现
  • 十、finally 办法的实现
  • 十一、catch办法的实现
  • Promise全副代码整合

一、Promise外围逻辑实现

首先剖析其原理

  1. promise就是一个类<br/>

在执行类的时候须要传递一个执行器进去，执行器会立刻执行

  1. Promise中有三种状态，别离为胜利-fulfilled 失败-rejected 期待-pending<br/>
    pending -> fulfilled<br/>
    pending -> rejected<br/>
    一旦状态确定就不可更改
  2. resolve 和 reject函数是用来更改状态的<br/>

resolve：fulfilled<br/>
reject：rejected

  1. then办法外部做的事件就是判断状态

如果状态是胜利，调用胜利回调函数<br/>
如果状态是失败，就调用失败回调函数<br/>
then办法是被定义在原型对象中的

  1. then胜利回调有一个参数，示意胜利之后的值；then失败回调有一个参数，示意失败后的起因

<PS:本文myPromise.js是源码文件，promise.js是应用promise文件>

// myPromise.js
// 定义成常量是为了复用且代码有提醒
const PENDING = 'pending' // 期待
const FULFILLED = 'fulfilled' // 胜利
const REJECTED = 'rejected' // 失败
// 定义一个构造函数
class MyPromise {
  constructor (exector) {
    // exector是一个执行器，进入会立刻执行，并传入resolve和reject办法
    exector(this.resolve, this.reject)
  }

  // 实例对象的一个属性，初始为期待
  status = PENDING
  // 胜利之后的值
  value = undefined
  // 失败之后的起因
  reason = undefined

  // resolve和reject为什么要用箭头函数？
  // 如果间接调用的话，一般函数this指向的是window或者undefined
  // 用箭头函数就能够让this指向以后实例对象
  resolve = value => {
    // 判断状态是不是期待，阻止程序向下执行
    if(this.status !== PENDING) return
    // 将状态改成胜利
    this.status = FULFILLED
    // 保留胜利之后的值
    this.value = value
  }

  reject = reason => {
    if(this.status !== PENDING) return
    // 将状态改为失败
    this.status = REJECTED
    // 保留失败之后的起因
    this.reason = reason
  }

  then (successCallback, failCallback) {
    //判断状态
    if(this.status === FULFILLED) {
      // 调用胜利回调，并且把值返回
      successCallback(this.value)
    } else if (this.status === REJECTED) {
      // 调用失败回调，并且把起因返回
      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)
 })

二、在 Promise 类中退出异步逻辑

下面是没有通过异步解决的，如果有异步逻辑加进来，会有一些问题

//promise.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
  // 主线程代码立刻执行，setTimeout是异步代码，then会马上执行，
  // 这个时候判断promise状态，状态是pending，然而之前并没有判断期待这个状态
  setTimeout(() => {
    resolve('success')
  }, 2000); 
 })

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

上面批改这个代码

// 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
  // 定义一个胜利回调参数
  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
    // 判断胜利回调是否存在，如果存在就调用
    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，如果存在就调用
    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 {
      // 期待
      // 因为并不知道状态，所以将胜利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

三、实现 then 办法屡次调用增加多个处理函数

promise的then办法是能够被屡次调用的。

这里如果有三个then的调用，

  • 如果是同步回调，那么间接返回以后的值就行；
  • 如果是异步回调，那么保留的成功失败的回调，须要用不同的值保留，因为都互不雷同。

之前的代码须要改良。

//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)
})

保留到数组中，最初对立执行

// 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
  // 定义一个胜利回调参数，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
    // 判断胜利回调是否存在，如果存在就调用
    // 循环回调数组. 把数组后面的办法弹出来并且间接调用
    // shift办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，如果存在就调用
    // 循环回调数组. 把数组后面的办法弹出来并且间接调用
    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 {
      // 期待
      // 将胜利回调和失败回调都保留在数组中
      this.successCallback.push(successCallback)
      this.failCallback.push(failCallback)
    }
  }
}

module.exports = MyPromise

四、实现then办法的链式调用

  • then办法要链式调用那么就须要返回一个promise对象，
  • then办法的return返回值作为下一个then办法的参数
  • then办法还一个return一个promise对象，那么如果是一个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) => {
// 目前这里只解决同步的问题
    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办法返回第一个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 的值时一般值还是promise对象
        // 如果是普通纸 间接调用resolve
        // 如果是promise对象 查看promise对象返回的后果
        // 再依据promise对象返回的后果 决定调用resolve还是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是不是其实例对象
  if(x instanceof MyPromise) {
    // promise 对象
    // x.then(value => resolve(value), reason => reject(reason))
    // 简化之后
    x.then(resolve, reject)
  } else{
    // 一般值
    resolve(x)
  }
}

module.exports = MyPromise

五、then办法链式调用辨认 Promise 对象自返回

如果then办法返回的是本人的promise对象，则会产生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>

所以为了防止这种状况，咱们须要革新一下then办法

// 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) {
        // 因为new Promise须要执行实现之后才有promise2，同步代码中没有pormise2，
        // 所以这部分代码须要异步执行
        setTimeout(() => {
          let x = successCallback(this.value)
          //须要判断then之后return的promise对象和原来的是不是一样的，
          //判断x和promise2是否相等，所以给resolvePromise中传递promise2过来
          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) {
  // 如果相等了，阐明return的是本人，抛出类型谬误并返回
  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>

六、捕捉谬误及 then 链式调用其余状态代码补充

目前咱们在Promise类中没有进行任何解决，所以咱们须要捕捉和处理错误。

1. 捕捉执行器的谬误

捕捉执行器中的代码，如果执行器中有代码谬误，那么promise的状态要弄成谬误状态

// myPromise.js
constructor (exector) {
    // 捕捉谬误，如果有谬误就执行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('执行器谬误')
})
 
promise.then(value => {
  console.log(1)
  console.log('resolve', value)
}, reason => {
  console.log(2)
  console.log(reason.message)
})

//2
//执行器谬误

2. then执行的时候报错捕捉

// myPromise.js
then (successCallback, failCallback) {
    let promise2 = new Promise((resolve, reject) => {
        if(this.status === FULFILLED) {
            setTimeout(() => {
                // 如果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('执行器谬误')
 })
 
// 第一个then办法中的谬误要在第二个then办法中捕捉到
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. 谬误之后的链式调用

// 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进行解决
        } else if (this.status === REJECTED) {
            setTimeout(() => {
                // 如果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('执行器谬误')
 })
 
 // 第一个then办法中的谬误要在第二个then办法中捕捉到
 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
// 执行器谬误
// 3
// 100

4. 异步状态下链式调用

还是要解决一下如果promise外面有异步的时候，then的链式调用的问题。

// myPromise.js
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
class MyPromise {
  constructor (exector) {
    // 捕捉谬误，如果有谬误就执行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，因为上面push到外面的时候曾经解决好了
    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，因为上面push到外面的时候曾经解决好了
    while(this.failCallback.length) this.failCallback.shift()()
  }

  then (successCallback, failCallback) {
    let promise2 = new Promise((resolve, reject) => {
      if(this.status === FULFILLED) {
        setTimeout(() => {
          // 如果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(() => {
          // 如果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 {
        // 解决异步的胜利谬误状况
        this.successCallback.push(() => {
          setTimeout(() => {
            // 如果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(() => {
            // 如果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) => {
  // 一个异步办法
  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

七、将then办法的参数变成可选参数

then办法的两个参数都是可选参数，咱们能够不传参数。
上面的参数能够传递到最初进行返回

var promise = new Promise((resolve, reject) => {
      resolve(100)
    })
    promise
      .then()
      .then()
      .then()
      .then(value => console.log(value))
// 在控制台最初一个then中输入了100

// 这个相当于
promise
  .then(value => value)
  .then(value => value)
  .then(value => value)
  .then(value => console.log(value))

所以咱们批改一下then办法

// myPromise.js
then (successCallback, failCallback) {
    // 这里进行判断，如果有回调就抉择回调，如果没有回调就传一个函数，把参数传递
    successCallback = successCallback ? successCallback : value => value
    // 谬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之后

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

// succ

reject之后

// 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

八、promise.all办法的实现

promise.all办法是解决异步并发问题的

// 如果p1是两秒之后执行的，p2是立刻执行的，那么依据失常的是p2在p1的后面。
// 如果咱们在all中指定了执行程序，那么会依据咱们传递的程序进行执行。
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"]
})

剖析一下：

  • all办法接管一个数组，数组中能够是一般值也能够是promise对象
  • 数组中值得程序肯定是咱们失去的后果的程序
  • promise返回值也是一个promise对象，能够调用then办法
  • 如果数组中所有值是胜利的，那么then外面就是胜利回调，如果有一个值是失败的，那么then外面就是失败的
  • 应用all办法是用类间接调用，那么all肯定是一个静态方法
//myPromise.js
static all (array) {
    //  后果数组
    let result = []
    // 计数器
    let index = 0
    return new Promise((resolve, reject) => {
      let addData = (key, value) => {
        result[key] = value
        index ++
        // 如果计数器和数组长度雷同，那阐明所有的元素都执行结束了，就能够输入了
        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，如果是resolve就把值增加到数组中去，如果是谬误就执行reject返回
          current.then(value => addData(i, value), reason => reject(reason))
        } else {
          // 一般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"]
})

九、Promise.resolve办法的实现

  • 如果参数就是一个promise对象，间接返回，如果是一个值，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) {
    // 如果是promise对象，就间接返回
    if(value instanceof MyPromise) return value
    // 如果是值就返回一个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 之后输入 p1

十、finally 办法的实现

  • 无论以后最终状态是胜利还是失败，finally都会执行
  • 咱们能够在finally办法之后调用then办法拿到后果
  • 这个函数是在原型对象上用的
// myPromise.js
finally (callback) {
    // 如何拿到以后的promise的状态，应用then办法，而且不管怎样都返回callback
    // 而且then办法就是返回一个promise对象，那么咱们间接返回then办法调用之后的后果即可
    // 咱们须要在回调之后拿到胜利的回调，所以须要把value也return
    // 失败的回调也抛出起因
    // 如果callback是一个异步的promise对象，咱们还须要期待其执行结束，所以须要用到静态方法resolve
    return this.then(value => {
      // 把callback调用之后返回的promise传递过来，并且执行promise，且在胜利之后返回value
      return MyPromise.resolve(callback()).then(() => value)
    }, reason => {
      // 失败之后调用的then办法，而后把失败的起因返回进来。
      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
// 两秒之后执行p2 reject

十一、catch办法的实现

  • catch办法是为了捕捉promise对象的所有谬误回调的
  • 间接调用then办法，而后胜利的中央传递undefined，谬误的中央传递reason
  • catch办法是作用在原型对象上的办法
// 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全副代码整合

// 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) {
    // 如何拿到以后的promise的状态，应用then办法，而且不管怎样都返回callback
    // 而且then办法就是返回一个promise对象，那么咱们间接返回then办法调用之后的后果即可
    // 咱们须要在回调之后拿到胜利的回调，所以须要把value也return
    // 失败的回调也抛出起因
    // 如果callback是一个异步的promise对象，咱们还须要期待其执行结束，所以须要用到静态方法resolve
    return this.then(value => {
      // 把callback调用之后返回的promise传递过来，并且执行promise，且在胜利之后返回value
      return MyPromise.resolve(callback()).then(() => value)
    }, reason => {
      // 失败之后调用的then办法，而后把失败的起因返回进来。
      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

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