How to chain an async ajax function in a loop

Question

I would like to chain an asynchronous ajax function that is called several times within a loop. Why? Because that function is to upload files but I want it to serially upload files rather than uploading all of them at once. I don't want to use async false because I want the progress updated on the DOM.

for (var i = 0; i < files.length; i++) {
         var fd = new FormData();

         fd.append('file', files[i]);
         fd.append('galleryid', galleryid);

        sendFileToServer(fd); //chain this function call
    }

function sendFileToServer(formData)
{
    var uploadURL ="includes/ajax/images/uploadImagePage.php"; //Upload URL
    var extraData ={}; //Extra Data.
    return jqXHR=$.ajax({
            xhr: function() {
            var xhrobj = $.ajaxSettings.xhr();
            if (xhrobj.upload) {
                    xhrobj.upload.addEventListener('progress', function(event) {
                        var percent = 0;
                        var position = event.loaded || event.position;
                        var total = event.total;
                        if (event.lengthComputable) {
                            percent = Math.ceil(position / total * 100);
                        }
                        //Set progress
                       $('#uploadImageResponse').html(percent+'%');
                    }, false);
                }
            return xhrobj;
        },
    url: uploadURL,
    type: "POST",
    async:true,
    contentType:false,
    processData: false,
        cache: false,
        data: formData,
        success: function(data){
            status.setProgress('Upload completed. 100%.');

        }
    }); 


}

I don't understand how jquery deferred objects work.

Answer 1

There are a number of different approaches for serializing multiple requests like this. Here's one way that uses a manual iteration instead of the for loop so you only advance to the next iteration when the previous one has completed.

Manual Iteration

function sendAllFiles(files) {
    var index = 0;

    function next() {
        var fd;
        if (index < files.length) {
            fd = new FormData();
            fd.append('file', files[index]);
            fd.append('galleryid', galleryid);
            ++index;
            // send this file and when done, do the next iteration
            sendFileToServer(fd).then(next);
        } else {
            // all files are done now
        }
    }
    // start the first iteration
    next();
}

---

Chained promises using .reduce()

And, here's a different design pattern using .reduce() and chained promises:

files.reduce(function(p, item) {
    return p.then(function() {
        var fd = new FormData();
        fd.append('file', item);
        fd.append('galleryid', galleryid);
        return sendFileToServer(fd);
    });
}, $.Deferred().resolve()).then(function() {
    // all files are done now
});

Answer 2

You could always try this...

function step1() { 
       updatePercentage(); 
       $.ajax({ ... , // fill in these details
                  success: step2 }); }
function step2(data) { 
      updatePercentage(); 
       $.ajax({ ... ,
                success: step3 });
}
function step3(data) { 
       updatePercentage(); 
       $.ajax({ ... ,
                success: step4?? });
}

This could be refactored down. I'll leave that for you though :)