JavaScript Common Pattern

Anonymous Closures

This is the fundamental construct that makes it all possible, and really is the single best feature of JavaScript. We’ll simply create an anonymous function, and execute it immediately. All of the code that runs inside the function lives in a closure, which provides privacy and state throughout the lifetime of our application.

(function () {
	// ... all vars and functions are in this scope only
	// still maintains access to all globals
}());

Global Import

JavaScript has a feature known as implied globals. Whenever a name is used, the interpreter walks the scope chain backwards looking for a var statement for that name. If none is found, that variable is assumed to be global. If it’s used in an assignment, the global is created if it doesn’t already exist. This means that using or creating global variables in an anonymous closure is easy. Unfortunately, this leads to hard-to-manage code, as it’s not obvious (to humans) which variables are global in a given file.

Luckily, our anonymous function provides an easy alternative. By passing globals as parameters to our anonymous function, we import them into our code, which is both clearer and faster than implied globals. Here’s an example:

(function ($, YAHOO) {
	// now have access to globals jQuery (as $) and YAHOO in this code
}(jQuery, YAHOO));

Module Export

Sometimes you don’t just want to use globals, but you want to declare them. We can easily do this by exporting them, using the anonymous function’s return value. Doing so will complete the basic module pattern, so here’s a complete example:

var MODULE = (function () {
	var my = {},
		privateVariable = 1;

	function privateMethod() {
		// ...
	}

	my.moduleProperty = 1;
	my.moduleMethod = function () {
		// ...
	};

	return my;
}());

Notice that we’ve declared a global module named MODULE, with two public properties: a method namedMODULE.moduleMethod and a variable named MODULE.moduleProperty. In addition, it maintainsprivate internal state using the closure of the anonymous function.