jQuery’s deferred objects can be tough to get your head around. Specifically learning the difference between always , done , fail , and then . Let’s go over them really quick and then dive into some examples!

Some Examples

Let’s cement this with some easy examples!

Always

$.get( "www.yoursite.com/api" ).always(function() {
  console.log('this will run whether the $.get fails or succeeds');
$.get( "www.yoursite.com/api" ).done(function() {
  console.log('this will run if the $.get succeeds');
$.get( "www.yoursite.com/api" ).fail(function() {
  console.log('this will run if the $.get fails');
$.get( "www.yoursite.com/api" ).then(
    function() {
      console.log('this will run if the $.get succeeds');
    }, function() {
        console.log('this will run if the $.get fails');
    }, function() {
        console.log('this will run if the deferred generates a progress update');
Hope that helps! There are a bunch of other cool methods you should check out! Let me know in the comments if you have any questions.
					
.then() doesn’t actually seem to be a shorthand for the others. It seems to function a little differently.
With .then(), you can return a new Deferred from your callbacks, and effectively chain your deferreds, whereas .done() doesn’t allow deferred chaining.
An example:
var a = $.Deferred();

var b = $.Deferred();
a.done(function() { console.log(“a1”); return b; }).done(function() { console.log(“a2”); });

b.done(function() {console.log(“b1”);});
a.resolve();

// Prints “a1” and “a2”
b.resolve();

// Prints “b1”
whereas:
var a = $.Deferred();

var b = $.Deferred();
a.then(function() { console.log(“a1”); return b; }).then(function() { console.log(“a2”); });

b.then(function() {console.log(“b1”);});
a.resolve();

// Prints “a1”
b.resolve();

// Prints “b1” and “a2”
The second .then() on the a is actually chained onto the return Deferred object from the first .then().
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