The modern way to deep-merge objects. It cleanly attaches itself to all Objects without showing up in `for...in` loops and modifies the object being copied into.

extend.js

/*
Copyright (c) 2014 Rohit Vighne
License: The MIT License (MIT)
*/

Object.defineProperty(Object.prototype, "extend", {
	value: function() {
		for (var i = arguments.length; i--;) {
			var obj = arguments[i];
			for (var j in obj) {
				var exists = j in this;
				if (obj[j] instanceof Object && exists) {
					this[j].extend(obj[j]);
				} else if (!exists) {
					this[j] = obj[j];
				}
			}
		}

		return this;
	},

	// Match the native functions as closely as possible
	configurable: true, enumerable: false, writable: true,
});