jQuery's deferred Show archive.org snapshot objects behave somewhat like standard promises Show archive.org snapshot , but not really.

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One of many subtle differences is that there are two ways to chain callbacks to an async functions.

The first one is done, which only exists in jQuery:

$.ajax('/foo').done(function(html) {
  console.debug("The server responded with %s", html);
});

There is also then, which all promise libraries have:

$.ajax('/foo').then(function(html) {
  console.debug("The server responded with %s", html);
});

However, .done() and .then() do not behave the same. In particular, if a standard promise callback returns another promise, this will delay the resolution of all later promises. jQuery's then behaves like this, but done does not. It is not possible for a done callback to delay the resolution of later callbacks.

Always prefer .then() so your code is compatible with the open promise standard ( Promises/A+ Show archive.org snapshot ).

Example

We want to request /foo and, as soon as we have the response, make a second request to /bar:

var promise = $.ajax('/foo').done(function() {
  return $.ajax('/bar');
});

Quiz: When does promise resolve?

Well, if you use done, it will resolve as soon as we have the response to /foo:

var promise = $.ajax('/foo').done(function() {
  return $.ajax('/bar'); // Returning does not actually do anything here.
});

promise.then(function() {
  console.debug("We have the response to /foo, but /bar is still loading");
});

If we use then instead, it will only resolve once we have responses to both /foo and /bar:

var promise = $.ajax('/foo').then(function() {
  return $.ajax('/bar');
});

promise.then(function() {
  console.debug("We have the responses to both /foo and /bar");
});

Enjoy.