JavaScript callback style - Moving to promises

Question

I'm trying to work out how I can do my JavaScript callbacks in a different style.

My current style of callback, passes the callback into the function, and the function calls the callback. For example:

Function call

doSomething(function(data){
   console.log(data);
});

Function

function doSomething(callback) {
    // Call the callback
    callback("someData");
}

This is the current style of callback that I'm using. But I've seen libraries that do their callbacks in a different style. For example:

Function call

doSomething().success(function(data){
   console.log(data);
});

How does this work, and could anybody outline a simple example for me? Thanks for your time.

Answer 1

That is an implementation of the promise library. jQuery has an implementation called deferreds and another one is Q.

doSomething would look like something like this, using jQuery.

function doSomething() {
   var dfd = $.deferred();
   // do your logic
   // eventually call dfd.resolve();
   return dfd.promise();
}

Then calling it, use

doSomething().then(function() {
   // do something else
});

What is nice about this pattern, is that you can have multiple callbacks and error callbacks.