Question

Say you have the following piece of code:

function someProcess() {
    var deferred = $.Deferred();

    apiCall(function (recvData) {
        deferred.resolveWith(null, [recvData]);
    });

    return deferred.promise();
}

function mainFunction() {
    $.when(someProcess())
        .then(someOtherProcess);
}

In this example I only need to wait for a single deferred to be resolved. In this case, what is the difference (if any) between writing the second function as above versus writing it like this:

function mainFunction() {
    someProcess()
        .then(someOtherProcess);
}

I mean, I like writing it the first way because it makes it clear that we’re using jQuery deferred objects, but I’m curious if it’s necessary in this case.

edit: I fixed a typo in the then() call. Thanks for catching that.

edit: Thanks for the answer nrabinowitz. I think you have nailed the points that I was not sure about in regard to using when() vs. using a raw jQuery deferred object instance. I went and fixed my code again to return a promise instead of the entire deferred object. That is how I do it in my actual code right now, just forgot to add it here.

Answer 1

I haven’t used custom Deferred objects, but my understanding is that there are two differences in your examples:

• Using $.when() allows you to arbitrarily add more deferred objects to be handled by your handlers. Obviously, not a big deal, since you’d have to change that line of code either way to add more deferreds.

• $.when() only gets the Promise object, not the whole deferred object, which if I understand correctly is a consistency measure to hide the deferred’s state-changing methods (e.g. resolve()), only exposing handler hooks and state inspection methods. This seems like good practice, but in your example you could do the same thing by calling someProcess().promise() instead of $.when(someProcess()).

So I think you’re right – both methods will work, but the main benefit of using $.when() is to make the code more legible and set expectations appropriately for other programmers. Using either $.when() or deferred.promise() will additionally help protect against some less competent coder messing with the state of your deferred in a confusing way.