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OOjs (short for "Object-oriented JavaScript") is a JavaScript library for working with objects. Features include inheritance , mixins , static inheritance and additional utilities for working with objects and arrays. It also provides an EventEmitter mixin for event-driven programming, and a factory pattern for decoupling consumers of functionality from any particular class's implementation.



OO.inheritClass( childClass, parentClass )

Extend prototype and static methods and properties of child constructor from a parent constructor.

OO.mixinClass( childClass, parentClass )

Copy prototype and static methods and properties to a child constructor from a parent constructor.

OO.initClass( baseClass )

Initialise a base class to be inherited by, or mixed into, a child class. Only required for classes that do not inherit or mixin other classes.


Allow event handlers to be attached, called when events occur, and detached.


Provide a simple map interface for associating arbitrary data with a symbolic name.


Registry of classes with instantiation abstraction.



See the API documentation for a complete reference of available utilities.



OOjs is optimised for modern ECMAScript 5 environments. The source code is compatible with the older ECMAScript 3 engine (such as in older versions of Internet Explorer, broadly speaking IE8 and before), but users need a polyfill to provide required ECMAScript 5 methods.

It is packaged to be run in either Web browsers or Node.js.

Extending OOjs classes in newer versions of ECMAScript


MediaWiki's ResourceLoader treats JavaScript code as ES6 by default since MediaWiki 1.41. ES6 supports the class syntax, so you may declare your extensions of OOjs (and OOUI ) classes using the new syntax. For example:

class MyDialog extends OO.ui.MessageDialog {
	static get name() {
		return 'myDialog';

	constructor(config) {
		// ...

	initialize() {
		// ...

However, you will then need to pass such a class through a helper function like this:

function es6ClassToOoJsClass(targetClass) {
	const originClass = Object.getPrototypeOf(targetClass);
	targetClass.static = Object.create(originClass.static);
		.filter((key) => key !== 'static')
		.forEach((key) => {
			const descriptor = Object.getOwnPropertyDescriptor(targetClass, key);
			if (descriptor.enumerable || descriptor.get) {
				targetClass.static[key] = targetClass[key];
	targetClass.parent = targetClass.super = originClass;
	return targetClass;

It assumes that the class has no static properties with a getter whose output is not fixed (which is unlikely).

If you're building your JavaScript project with a transpiler like Babel, you can also use the most modern syntax, with static properties declared as such:

	static name = 'myDialog';



In the presence of a module system, such as in Node.js, OOjs exports all of its classes and methods. In other environments, such as Web browsers, a global variable is created named OO. You may access OO the same way you would access jQuery or mediaWiki.

If you are using eslint, you should add the following to your .eslintrc.json file.

"globals": {
    "OO": "readonly"

See also