Order of calling "then" and "success" in jQuery deferred objects

Question

I am loading some data through a $.get method. I have used the a success function in the get call. But I'm also using a when/then construction.

Do both callbacks get called, and if so, which occurs first and why?

$.when(
  $.get("test.html", functionA )
).then( functionB );

If successful, is this guaranteed to call functionA, then functionB? Or is there a situation where B might occur before A, or can something in functionA ever prevent functionB from being called? I couldn't find the answer in the jQuery docs.

Answer 1

Summary: The functionA will be called earlier, than the functionB in your example. The success and error callbacks from jQuery.ajax settings (or passed to $.get in your example) will be called earlier, than any callback attached to the returned jqxhr object later. Well, today :-)

Details: Documentation of jQuery.ajax() does not mention this. You'd better avoid depending on it, if you can.

As long as you develop with a particular jQuery version, you can inspect its implementation and assume, that it doesn't change. If you upgrade, you should check, if the assumption is still valid.

For example, latest releases in all supported branches of jQuery (1.12.4, 2.2.4 and 3.1.1) register the success and error callbacks from jQuery.ajax settings before the caller gets the jqxhr object and can register their callbacks. The jqxhr object is a Deferred Object which guarantees, that handlers attached to its resolution or rejection are executed in the order of their attaching. You can confirm it in the documentation of deferred.done(), for example. It means, that, success and error callbacks from jQuery.ajax settings will be always called earlier, than any callback attached to the returned jqxhr object later.

See jQuery.ajax 3.1.1 sources:

// Install callbacks on deferreds
completeDeferred.add( s.complete );
jqXHR.done( s.success );
jqXHR.fail( s.error );

I admit, that changing the order of execution is rather unlikely in future jQuery updates. That's why I think, that depending on it would be pretty stable.