SOURCE

//自定义请求函数
var request = url => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(`任务${url}完成`)
        }, 1000)
    }).then(res => {
        console.log('外部逻辑', res);
    })
}

//添加任务
function addTask(url){
    let task = request(url);
    pool.push(task); 
    task.then(res => {
        //请求结束后将该Promise任务从并发池中移除
        pool.splice(pool.indexOf(task), 1);
        console.log(`${url} 结束，当前并发数：${pool.length}`);
        url = urls.shift();
        // //每当并发池跑完一个任务，就再塞入一个任务
        if(url !== undefined){
            addTask(url);
        }
    })
}

let urls =  ['bytedance.com','tencent.com','alibaba.com','microsoft.com','apple.com','hulu.com','amazon.com'] // 请求地址
let pool = []//并发池
let max = 3 //最大并发量
//先循环把并发池塞满
while (pool.length < max) {
    let url = urls.shift();
    addTask(url)
}