promise_fetch.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>


const  multiRequest = (urls, maxNum) => {


   return  new Promise((resolve) => {
        const length = urls.length;
        let count = urls.length;
        let res = new Array(length);
    

        let handler = index => result => {
            res[index] = result;
            //console.log(res);
            maxNum++;
            if(count === 0){
                //console.log(res);
                resolve(res);
            }else{
                mockFetch(urls.shift()).then(handler(length-count));
                maxNum--;
                count--;
            }
        }

        while(maxNum > 0){
            mockFetch(urls.shift()).then(handler(length-count));
            maxNum--;
            count--;
        }


    });


}


function mockFetch(url) {
return new Promise((resolve) => {
  let start = new Date()
  setTimeout(() => {
    resolve(`start: ${start};end: ${new Date()}`)
  }, 0);
})
}


setTimeout(function(){
multiRequest([1,2,3,4,5,6,7,8,9], 3).then(x => console.log(x))

},5000);


    </script>


</body>
</html>