Rationale for CoffeeScript's 'Fat Arrow' syntax

I gave a CoffeeScript presentation last night at our local Groovy Users of Minnesota meet up. One of the questions that came up last night had to do with CoffeeScript's fat arrow (=>) syntax, which will both define a new function and bind the new function to the current value of this. This is super helpful for callback-based libraries, such as jQuery.

I was building a Backbone.View today (in JavaScript) and had to deal with this binding. In my JavaScript, I wanted to call fadeOut on the view's el property (which happens to be a jQuery wrapped element). Without using the jQuery proxy method to bind this appropriately, my statement of 'this.el.fadeOut();' will not work. The this reference is no longer the view at the point I'm using it in the anonymous function; the context for this has changed within the anonymous function declaration. The correct JavaScript code is below.

var MyView = Backbone.View.extend({

    initialize: function() {
        this.template = _.template($('#my-template').html(), this.model.toJSON());
        this.render();
    },

    render: function() {
        this.el.html(this.template);
        return this;
    },

    events: {
        "click button#doSomethingButton": "doSomething",
    },

    doSomething: function(e) {
        this.model.set({someValue: $('#someValueTextField').val()});
        var promise = this.model.doSomethingOnModel();
        promise.done($.proxy(function() {
            this.el.fadeOut();
        }, this)).fail(function() {
            alert('Failed to check sequence uniqueness.');
        });
    }                                               
});

So how does this relate to CoffeeScript? Well, the fat arrow operator is performing the proxying of the 'this' reference for you. If I wrote the above in CoffeeScript, I could write the done callback as:

…
promise.done => @el.fadeOut()
…

Chaining the done and the fail callbacks would necessitate the use of parentheses, but still very succinct. Score one for CoffeeScript!