< del > Promise.all , or recursively call and delete the array item until it is 0, which is even easier if you can use async/await . < / del >
none of the above is correct. The correct thing to do is to ask your backend to return url . What does it mean to have more than ten requests for one piece of data?

just use Synchronize

async: false,

it is recommended to change it to post request
request parameters. All id are grouped into an array in the request parameters, and all url are returned uniformly at the backend

var urlList=[]
var idList = [1,2,3,4,5]
$.ajax({
    url:'api/v1/getMusicByIds,
    type:'post',
    data:idList,
    success:(res)=>{
        urlList = [...res.url]
    } 
})

if the asynchronous request is nested inside the loop, the asynchronous result will not be returned at the end of the loop. Try a layer of closure on the outside. If forEach does not work, replace it with for,. I have used asynchronous request in for loop. I do not know whether forEach will work without forEach,.

var urlList=[]
arr.forEach((item)=>{
    (function(){
        $.ajax({
        url:'api/v1/getMusicById?id='+item.id,
        type:'get',
        success:(res)=>{
            urlList.push(res.url)
        }
        
        })    
    })()
})
console.log(urlList)

I think there is something wrong with your needs

generally, url that requires music is needed when playing, and only one song is played at a time, so you can execute an ajax request to get url through id

set a variable index=0; after each success and then determine whether arr [index] has a value. In some cases, it is a good choice to continue ajax, recursion

is a problem of requirements. Even if the function is done, it is not possible to send so many requests at once. The server explodes and changes it to the request list