同步迭代（Symbol.iterator、for of）

一个 Object 要想是可迭代的，需要有 Symbol.iterator 属性

function buildIterator() {  const timeList = [2000, 1000, 3000, 5000, 4000]  return {    next() {      return {        done: timeList.length === 0,        value: timeList.shift()      }    }  }}function testSync() {  const begin = Date.now()  console.log(Date.now() - begin, 'begin')  const iteratableObj = {    [Symbol.iterator]: buildIterator  }  for (let item of iteratableObj) {    console.log(Date.now() - begin, item)  }  console.log(Date.now() - begin, 'end')}testSync()

运行结果：

异步迭代（Symbol.asyncIterator、for await...of）

一个 Object 要想是异步可迭代的，需要有 Symbol.asyncIterator 属性

实现如下功能：给定一个数组 [2000, 1000, 3000, 5000, 4000]，按顺序按值对应的时长打印出来

function buildAsyncIterator() {  const timeList = [2000, 1000, 3000, 5000, 4000]  return {    next() {      return new Promise((resolve, reject) => {        setTimeout(() => {          resolve({            done: timeList.length === 0,            value: timeList.shift()          })        }, timeList[0])      })    }  }}async function testAsync() {  const begin = Date.now()  console.log(Date.now() - begin, 'begin')  const asyncIteratableObj = {    [Symbol.asyncIterator]: buildAsyncIterator  }  for await (let item of asyncIteratableObj) {    console.log(Date.now() - begin, item)  }  console.log(Date.now() - begin, 'end')}testAsync()

运行结果：

Promise（async await、for of）

当然不用异步迭代也能实现

function buildPromise(time) {  return new Promise((resolve, reject) => {    setTimeout(() => {      resolve(time)    }, time)  })}async function testPro() {  const begin = Date.now()  console.log(Date.now() - begin, 'begin')  const timeList = [2000, 1000, 3000, 5000, 4000]  for (item of timeList) {    const time = await buildPromise(item)    console.log(Date.now() - begin, time)  }  console.log(Date.now() - begin, 'end')}testPro()

运行结果：

参考

for-await-of and synchronous iterables

Async iteration and generators

Asynchronous iteration

原文：https://juejin.cn/post/7099258513939464199