Chaining multiple done() callbacks to the same deferred promise

Question

In short, I would like to have one general callback that always fires in case of a successful ajax call, followed by separate callback functionality depending on where the method is invoked.

This seems to work. My question is if this is a correct use of the promise object and if it's safe to assume that multiple promise callbacks of the same type always stack sequentially?

var dfd = $.Deferred(),
    promise = dfd.promise();

promise.done(function(){
    console.log(1);
}).done(function(){
    console.log(2);
});

dfd.resolve();

http://jsfiddle.net/4ax4nxbh/

Answer 1

It's a correct and documented use of the deferred object in jQuery. The documentation clearly states:

Callbacks are executed in the order they were added.

It works differently in other promise libraries and generally .then is preferred to .done anyway (explained later in the answer). However given you're using jQuery promises it'll stack just fine sequentially if they're synchronous.

So the direct answer to your question is yes.

However, you can also do this with asynchronous code and have it chain better with .then:

promise.then(function(){
    console.log(1);
}).then(function(){
    console.log(2);
}).then(function(){
    return $.get(...);
}).then(function(){
    console.log(3); // this always executes after the $.get finishes.
});

Basically, done adds a handler and returns the same promise and .then returns a new promise chained from the last one. In general I'd only use .done to terminate chains and if you want to keep the return value (the argument of the function(){)