Extend Object or Array own prototype.

extend.js

function extendOwnPrototype(obj, prototype) {
	var oldPrototype 	= Object.getPrototypeOf(obj);
	var newPrototype 	= Object.create(oldPrototype);
	var newObject 		= Object.create(newPrototype);
	var length 		= Object.getOwnPropertyDescriptor(obj, 'length');

	for (var key in prototype) {
		Object.defineProperty(newPrototype, key, { 'value': prototype[key] });
	}
	for (var key in obj) {
		obj.hasOwnProperty(key) && (newObject[key] = obj[key]);
	}

	length && Object.defineProperty(newObject, 'length', length);

	return (newObject);
};

/* EXAMPLE */

var n = extendOwnPrototype(['val'], {
	'fn': function() { /* ... */ }
});

var n2 = extendOwnPrototype({'key': 'val'}, {
	'fn': function() { /* ... */ }
});