Deferred Async Tasks with jQuery

Question

I'm new to deferred classes in JavaScript and would like to implement a function which will loop through forms and submit them one by one.

Looks like Deferred classes are the way to accomplish this.

I tried following this answer, but for some reason my implementation starts, waits 3 seconds and completes. I want it to show a different form name every 3 seconds until it's done with all the forms then complete.

What am I doing wrong? JSFIDDLE

function syncAll() {
        
  var promises = [];
  var forms = [
    {'name':'form 1'},
    {'name':'form 2'},
    {'name':'form 3'}, 
    {'name':'form 4'}];

  $.each(forms, function (index, value) {
    var def = new $.Deferred();
    setTimeout(function () {
      $("#output").html("Syncing: " + value.name);
      def.resolve({ 'message': 'finito!' });
    }, 3000);
    promises.push(def);

  });

  return $.when.apply(undefined, promises).promise();
}
    
    
    $.when(syncAll()).done(function(response){
        $("#output").html(response.message);
    });
    /*
    syncAll().done(function (response) {
      $("#output").html(response.message);
    }));
    */

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="output">Start</div>

Answer 1

JSFiddle: https://jsfiddle.net/TrueBlueAussie/v6cgak1u/2/

This uses a promise = promise.then(functionReturningNextPromise) pattern:

function syncAll() {
    var promise = $.when();  // Start with a resolved promise.
    var forms = [
      {'name':'form 1'},
      {'name':'form 2'},
      {'name':'form 3'}, 
      {'name':'form 4'}];

    $.each(forms, function (index, value) {
        promise = promise.then(function(){
            var def = $.Deferred();
            setTimeout(function () {
                $("#output").html("Syncing: " + value.name);
                def.resolve({ 'message': 'finito!' });
            }, 3000);
            return def.promise();
        });
    });
    return promise;
}


$.when(syncAll()).done(function(response){
    $("#output").html(response.message);
});

This could be improved a bit by replacing the setTimeout and having a timed promise method instead that also chains together (still implemented using setTimeout).

Note that new is not required on $.Deferred()

EDIT :

Features of javascript and jQuery can be exploited to make the code slightly more concise :

function syncAll() {
    var forms = [
        {'name':'form 1'},
        {'name':'form 2'},
        {'name':'form 3'}, 
        {'name':'form 4'}
    ];
    return forms.reduce(function(promise, value) {
        return promise.then(function() {
            return $("#output").delay(1000).html("Syncing: " + value.name).promise();
        });
    }, $.when()).then(function() {
        return {'message': 'finito!'};
    });
}

syncAll().then(function(response) {
    $("#output").html(response.message);
});

This is essentially the same paradigm as above but exploits :

• 'array.reduce()' to build the promise chain from the forms array.
• jQuery's .delay() instead of setTimeout(), to implement delay.