CoffeeScript Fat Arrows

Lets look at the CoffeeScript usage of the fat arrow aka =>:

    foo = (hello)=>

Which translates to this javascript code:

    var foo;
    var __bind = function(fn, me){ return function(){ return fn.apply(me, arguments); }; };
    foo = __bind(function(hello) {
      return console.log(hello);
    }, this);


To understand what this does, we need to first understand what is function.apply.

From Mozilla Developer Network:

Calls a function with a given this value and arguments provided as an array.

So, each function can be specified of its this. Sounds weird? perhaps not.

In python, you need to pass self as a first argument to a method. Notice that by method I classically mean a function that is called on its object. python class MyClass: """A simple example class""" i = 12345 def f(self): return 'hello world'

In cases like working with Backbone, those functions that are triggered as a reaction to a DOM event, may be be run in the context of the DOM/window, simply because the browser will invoke them in that context.

When that happens this will no longer be the Backbone Model that the mentioned function belongs to -- that's because there is no notion of 'belongs to', there are no classes in Javascript; but there is a form of those in CoffeeScript.

Fat Arrow

Using a fat arrow gives a function a sense of being a proper method. When we look at the transformed Javascript code, we see that CoffeeScript generates a __bind function, which will take your function, and give you back a function that will call function.apply with a specified this parameters.

In all cases where you would use =>, the current instance of a class will be bound as this for that function you are defining. That closes up the relation between a function and an instance; a function that belongs to an instance, would take it as its this argument, completing its destiny as a proper method.

But I don't use CoffeeScript!

If you use plain Javascript, you can use underscore.js's _.bindAll function. Lets see what it does.

    _.bind = function(func, obj) {
        if (func.bind === nativeBind && nativeBind) return nativeBind.apply(func,, 1));
        var args =, 2);
        return function() {
          return func.apply(obj, args.concat(;

After analyzing the CoffeeScript translated version, we identify the function.apply idiom here pretty easily. However, underscore will call ECMAScript 5's native bind if available (nativeBind in the code).