Once - Can call a callback(eventHandlers mostly) only once, returns undefined the second time

once.js

//once
//There are times when you prefer a given functionality only happen once, similar to the way you'd use an onload event.  //This code provides you said functionality:

function once(fn, context) { 
	var result;

	return function() { 
		if(fn) {
			result = fn.apply(context || this, arguments);
			fn = null;
		}

		return result;
	};
}

// Usage
var canOnlyFireOnce = once(function() {
	console.log('Fired!');
});

canOnlyFireOnce(); // "Fired!"
canOnlyFireOnce(); // nada

//How it Works
//A function is passed to once(fn) as an argument. once() executes the callback function once and assignes null as the value of callback. Since, Javascript function are passed by reference, fn=null means the the original callback function also points to a null value. Hence the callback cannot be fired the second time