SOURCE

var urls = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];
const limit = 5;

function sendRequest(urls, limit , callback) {
    function _send (urls) {
        const url = urls.shift();
        if(url) {
            return new Promise((resolve, reject) => {
                setTimeout(()=>{
                    console.log(`当前发送：${url}`);
                    resolve(url);
                }, 2000)
            })
            .finally(() => {
                if(urls.length > 0) {
                    return _send(urls)
                }
            })
        }
       
    }
    let asyncList = [];
    while(limit--) {
        asyncList.push(_send(urls));
    }
    console.log('asyncList', asyncList)
    return Promise.all(asyncList).then(callback);
}

sendRequest(urls, limit, function() {
    console.log('all urls sended!')
});