>Javascript: modularity is working today but with some grinding, will get module syntax baked in only in ES6...

Modularity, sub-classing and cross-classing has always been possible with JavaScript;

eg: ActiveWidget is a JS library released in 2004;

on the combobox documentation you'll see:
- 4 parent classes
- 5 mixin classes
- about 50 properties
- about 200 methods
- a number of events

Here is the source code for that control (slightly obfuscated using '...' because of the copyright, while keeping the number of instructions):

AW.UI.Combo = AW.UI.List.subclass();

AW.UI.Combo.create = function(){

	AW.UI.ImageText.create.call(this);
	AW.UI.Input.create.call(this);
	AW.Templates.Combo.create.call(this);

	var obj = this.prototype;

	obj.setClass("ui", "combo");
	obj.setClass("input", "");

	obj.defineTemplate("popup", new AW.Templates....);

	obj.onCurrentItemChanged = function(i){
		...;
		...;
		...;
	};

	obj.setController("selection", {
		onKeyUp:			"...",
		onKeyDown:			"...",
		onItemClicked:		"..."
	});
};