Synchronous calls in javascript, but not handled synchronous

Question

I am having some issues with synchronous calls, and can't seem to understand exactly what's going wrong.

When debugging the code, it fills up the songTracks array perfectly, just until the return statement, where it is empty again.

Code sample:

function getAllSongIds(lijstId){
  var songTracks = [];
  $.ajax({
    url: "http://somehost.com/lists/"+lijstId+"/editions/",   
      dataType: "jsonp",
      async: false,
      success: function(json){

        for (i in json.editions) {
          $.ajax({
            url:"http://somehost.com/lists/"+lijstId+"/editions/"+json.editions[i].id,
            dataType:"jsonp",
            async: false,
            success: function(json2){
              for(j in json2.tracks){
                if(songTracks.indexOf(json2.tracks[j].id) === -1){
                  songTracks.push(json2.tracks[j].id);
                }
              }
            }
          })
        };
      }

    });
  alert(songTracks);
  return songTracks;
};

Welcome to the lands of callback hell, and looping XHR calls. I suggest you check out jQuery's promise feature. The issue is is that your code does not wait for the XHR to return, it moves on, so your success function is skewed by the delay — Sterling Archer, Aug 11 '14 at 19:10
[JSONP](https://en.wikipedia.org/wiki/JSONP) ***cannot*** be made sync by using `async:false` — Bergi, Aug 11 '14 at 19:11

score 7 · Accepted Answer · edited May 23 '17 at 10:26

JSONP is always asynchronous. The async: false setting is simply ignored. From the documentation:

[...] If you need synchronous requests, set this option to false. Cross-domain requests and dataType: "jsonp" requests do not support synchronous operation.

Why?

JSONP has actually nothing to do with Ajax. jQuery is just providing a single function to deal with both. JSONP is nothing else but appending a <script> element to the document. The browser will load these scripts asynchronously.

See How do I return the response from an asynchronous call? for possible (asynchronous) solutions.

Synchronous calls in javascript, but not handled synchronous

1 Answers1