Promise finally

这是一个promise实例原型链上的办法

Promise.prototype.finally = function(func) {  return this.then((value) => {    Promise.resolve(func()).then(() => value)  },  (reason) => {    Promise.resolve(func()).then(() => {throw reason})  })}

Promise.All

  if (!arr[Symbol.iterator]) throw 'param is not iterable';  const promiseArr = Array.from(arr).map(a => {    if (a instanceof Promise) {      return a;    } else {      return Promise.resolve(a);    }  })  if (promiseArr.length === 0) return Promise.resolve([]);  return new Promise((resolve, reject) => {    let result = [];    let count = promiseArr.length;    promiseArr.forEach((p, i) => {      p.then((res => {        result[i] = res;        count--;        if (count === 0) resolve(result);      }, (reason) => reject(reason)))    })  })

留神点

  1. 入参必须是可迭代的
  2. 参数转化为数组，且所有的项必须转化为Promise
  3. 返回值永远是一个Promise
  4. 入参如果长度是0 那间接返回Promise.resolve([]) 不然Promise.all([])返回的promise永远处于pending状态

Promise.race

const PromiseRace = (arr) => {  if (arr[Symbol.iterator] === undefined) return Promise.reject('param is not iterator')  const promiseArr = Array.from(arr).map(a => {    if (a instanceof Promise) {      return a;    } else {      return Promise.resolve(a);    }  })  return new Promise((resolve, reject) => {    promiseArr.forEach((p, i) => {      p.then((res => {        resolve(res);      }, (reason) => reject(reason)))    })  })}

留神 Promise.race([])返回的应该是一个pending状态的promise

Promise.any

const PromiseAny = (arr) => {  if (arr[Symbol.iterator] === undefined) return Promise.reject('param is not iterator')  const promiseArr = Array.from(arr).map(a => {    if (a instanceof Promise) {      return a;    } else {      return Promise.resolve(a);    }  })  if (promiseArr.length === 0) return Promise.reject(new AggregateError([],'All promises were rejected'))  return new Promise((resolve, reject) => {    const errors = [];    const count = promiseArr.length;    promiseArr.forEach((p, i) => {      p.then((res => {        resolve(res);      }, (reason) => {        errors[i] = reason;        count--;        if (count === 0) {          reject(new AggregateError(errors,'All promises were rejected'))        }      }))    })  })}

留神

  1. Promise.any 返回的谬误肯定是AggregateError类型的

Promise.allSettled

const PromiseAllSettled = (arr) => {  if (arr[Symbol.iterator] === undefined) return Promise.reject('param is not iterator')  const promiseArr = Array.from(arr).map(a => {    if (a instanceof Promise) {      return a;    } else {      return Promise.resolve(a);    }  })  if (promiseArr.length === 0) return Promise.resolve([])  return new Promise((resolve, reject) => {    const result = [];    const count = promiseArr.length;    promiseArr.forEach((p, i) => {      p.then((res => {        result[i] = {status: 'fulfilled', value: res};        count--;        if (count === 0) {          resolve(result)        }      }, (reason) => {        result[i] = {status: 'rejected', reason: reason};        count--;        if (count === 0) {          resolve(result)        }      }))    })  })}

返回内容的格局肯定是{status: 'fulfilled', value: res}或者{status: 'rejected', reason: reason}