一、不使用Promise.race

维护一个并发池pool，先将其填满，然后每当其中的promise完成后添加新的promise，直到并发队列清空。

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>

</body>
<script>
  let request = options =>{
    return new Promise((resolve,reject)=>{
      setTimeout(()=>{resolve(options.url)},options.delay)
    }).then((res)=>{
      console.log(`${options.url}任务完成`)
    })
  }
  let now = new Date()
  function addTask(options){
    let task = request(options)
    pool.push(task)
    task.then((res)=>{
      pool.splice(pool.indexOf(task),1)
      newOptions = requests.shift()
      if(newOptions){
        console.log(`${options.url}任务完成，耗时${(new Date()-now)/1000}秒,${newOptions.url}开始执行,预计耗时${newOptions.delay/1000}秒`)
        addTask(options)
      }else{
        console.log(`${options.url}任务完成，耗时${(new Date()-now)/1000}秒`)
      }
    })

  }
  let requests = [{
    url:'tencent',
    delay:1000
  },{
    url:'baidu',
    delay:3000
  },{
    url:'bytedance',
    delay:2000
  },{
    url:'tieba',
    delay:1000
  },{
    url:'bilibili',
    delay:1000
  },{
    url:'acfun',
    delay:1000
  },{
    url:'meituan',
    delay:1000
  }]
  let pool = []
  let max = 3
  while(pool.length<max){
    let options = requests.shift()
    addTask(options)
  }
  
</script>

</html>

二、使用Promise.race

原理类似，每当Promise.race()返回结果，就可以执行下一个promise了。

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  
</body>

<script>
   let request = options =>{
    return new Promise((resolve,reject)=>{
      setTimeout(()=>{resolve(options.url)},options.delay)
    }).then((res)=>{
      console.log(`${options.url}任务完成`)
    })
  }
  let now = new Date()

  function addTask(options){
    let task = request(options)
    pool.push(task)
    task.then((res)=>{
      console.log(`${options.url}任务完成，耗时${(new Date()-now)/1000}秒`)
      pool.splice(pool.indexOf(task),1)
    })
  }

  function run(race){
    race.then((res)=>{
      let newOptions = requests.shift()
      if(newOptions){
        console.log(`${newOptions.url}开始执行,预计耗时${newOptions.delay/1000}秒`)
        addTask(newOptions)
        run(Promise.race(pool))
      }

    })
  }


  let requests = [{
    url:'tencent',
    delay:1000
  },{
    url:'baidu',
    delay:3000
  },{
    url:'bytedance',
    delay:2000
  },{
    url:'tieba',
    delay:1000
  },{
    url:'bilibili',
    delay:1000
  },{
    url:'acfun',
    delay:1000
  },{
    url:'meituan',
    delay:1000
  }]
  let pool = []
  let max = 3
  while(pool.length<max){
    let options = requests.shift()
    addTask(options)
  }

  let race = Promise.race(pool) // 返回最先完成的promise
  run(race)
</script>
</html>

原文链接：https://blog.csdn.net/ZKH129/article/details/125607698