Example "subclass" of Error object

HttpError.js

// AMD module wrapper.
define(function () {

	function HttpError (msg, code) {
		var ex;

		msg = msg || 'Unknown HTTP error';
		ex = new Error(msg); // not all browsers let us inherit, so we do it this way
		ex.code = code;
		ex.name = 'HttpError'; // thx @cjno! (http://twitter.com/cjno/status/154959746582056960)
		ex.toString = function () {
			var code = typeof this.code == 'undefined' ? '' : '(' + this.code + ') ';
			return 'Error: ' + code + this.message;
		};

		return ex;
	}

	return HttpError;

});

usage.js

throw new HttpError('session timed out', 405);

/*
this has several advantages over `throw 'session timed out'`, including:

1. browsers will include a full stack trace
2. other data (e.g. status code) can be carried along
3. you can sniff for the type of error via ex.name;

I had some issues with several browsers when trying to inherit from the 
native Error object.  That would be even better because then we could do
this:

if (ex instanceof HttpError) { ... }

*/