Force jQuery Deferred to wait until Ajax complete in “then” handler

Question

I have situation where I believe I need to create a Deferred object with a "then" handler, but wait until the "then" handler has completed it's own promise before moving on.

The use case is a record object, and the above function is it's save method. The record object has an attribute called saveQueue, which is set to $.Deferred() on the record's instantiation. The resolve call on saveQueue was supposed to make sure the Deferred there is always executing every new handler attached to it as soon as it could. The idea being that you can call save several times on the record in short succession, but the calls will run one after another, and not overlap.

I am using a Deferred to enqueue Ajax calls, so that one does not run until the previous one call finished. However, from the same method, I want to return a Deferred that can be resolved/rejected by the jQuery Ajax object, like so:

 record.saveQueue = $.Deferred();

 self.save = function( record ){
    var deferredAction = $.Deferred();

    deferredAction.then(function() {
        return $.post("/example_save_endpoint");
    });

    record.saveQueue.always(function(){
      deferredAction.resolve();
    }).resolve();

    return deferredAction;
  }

However, when I use this code, the deferredAction promise always ends up resolved, presumably because the #then handler is returning a "pending" (and thus non-rejecting) promise. Is there any way to force the Deferred to wait until the Ajax promise is complete before resolving/rejecting? Or is there another, better way to thread this needle?

Answer 1

Your idea might work, but

• the queue must not be resolved using .resolve() every time the method is called, instead it should be initialised only with a resolved promise.
• to actually queue on the record.saveQueue, it needs to be changed (overwritten) on every method call, to represent the end of the latest request.

And we don't need any deferreds for that, as we can work with the promises that $.post returns.

So use this:

var emptyQueue = $.when(undefined); // an already fulfilled promise as the start
// equivalent: = $.Deferred().resolve().promise();

function startQueue() {
    return emptyQueue; // yes, this delibaretely returns a constant, the begin
                       // of the queue always looks the same (and is never mutated)
}

// every time you create a record, do
record.saveQueue = startQueue();

// and use that in your methods:
this.save = function(record) {
    var queuedRequestResult = record.saveQueue.then(function() {
        return $.post("/example_save_endpoint");
//      ^^^^^^ promises chain :-)
    });
    // Magic happens here:
    record.saveQueue = queuedRequestResult // we swap the previous queue promise for a new
                                           // one that resolves only after the request
      .then(startQueue, startQueue);       // and make sure it then starts with a fresh
                                           // queue, especially when the request failed
    //.then(null, startQueue) is similar, except unnecessarily remembering the last result
    return queuedRequestResult;
}

Answer 2

I would probably choose not to do it this way, but a deferred/promise can indeed be used as a queuing device.

You need a slight(?) variation of what you already tried.

self.queue = $.when();//A resolved promise, used to form a queue of functions in a .then() chain.

self.save = function(data) {
    var dfrd = $.Deferred();//A Deferred dedicated to this particular save.
    self.queue = self.queue.then(function() {
        return $.post("/example_save_endpoint", data) //Make the AJAX call, and return a jqXHR to ensure the downstream queue waits for this jqXHR to resolve/reject.
            .then(dfrd.resolve, dfrd.reject) //Resolve/reject the Deferred for the caller's benefit
            .then(null, function() {
                //Force failure down the success path to ensure the queue is not killed by an AJAX failure.
                return $.when();//Return a resolved promsie, for the queue's benefit.
            });
    });
    return dfrd.promise();//allow the caller to do something when the AJAX eventually responds
}

For explanation, see comments in code