Object Oriented Programming in JavaScript

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DocumentationZK Client-side ReferenceIntroductionObject Oriented Programming in JavaScript
Object Oriented Programming in JavaScript

JavaScript is not an object-oriented language, but ZK provides some utilities to enable object-oriented programming.

The JavaScript Package

Like Java, ZK's JavaScript classes are grouped into different packages. Similar to Java, the JavaScript code is loaded on demand, but it is loaded on per-package basis rather than per-class (i.e., the whole package is loaded if needed).

The dependence of the packages is defined in the so-called Widget Package Descriptor (aka., WPD). If it is about to load a package, all packages it depends will be loaded too.

Define a Package

A package is usually defined implicitly by the use of a WPD file, such as

<package name="zul.grid" language="xul/html" depends="zul.mesh,zul.menu">
	<widget name="Grid"/>
	<widget name="Row"/>
	<widget name="Rows"/>

You rarely need to define it explicitly, but, if you want, you could use zk.$package(String). For example,


Similarly, you could, though rarely needed, import a package by the use of zk.$import(String).

Notice that, if the package is not loaded yet, zk.$import(String) won't load the package but returns null.

Load Packages

To force one or multiple packages to load, you could use zk.load(String, Function). Since ZK loads the packages asynchronously, you cannot access any of the code right after the invocation of zk.load(String, Function). Rather, you should specify the code in the second argument as a function (Function). For example,

zk.load("zul.inp, zul.layout", function () { //load zul.inp and zul.layout
    new zul.layout.Hlayout({
        children: [new zul.inp.Textbox({value: 'foo'}]
   }); //Correct! zul.inp and zul.layout are both loaded
new zul.inp.Textbox({value: 'foo'}); //WRONG! zul.inp not loaded yet

Do After Load

If you have some code that should execute when a particular package is loaded, you could use zk.afterLoad(String, Function). Unlike zk.load(String, Function), it won't force the package(s) to load. Rather, it only registers a function that is called when the specified package(s) is loaded by others.

It is useful to customize the default behavior of widgets, since they might be loaded when your code is running. For example, we could customize SimpleConstraint as follows.

zk.afterLoad('zul.inp', function () {
  zu.inp.SimpleConstraint.prototype.validate = function (inp, val) {
    //...customized validation

Then, the above code can be evaluated even if the zul.inp package is not loaded yet.


If the customization requires a lot of codes and you prefer to put it in a separate package, you could use zk.depends(String, String) as follows.

zPkg.depends('zul.inp', 'com.foo');

which declares the zul.inp package depends on the com.foo package. In other words, com.foo will be loaded when zul.inp is loaded.

The JavaScript Class

The root of the class hierarchy is Object. To define a new class, you have to extend from it or one of the deriving classes.

Define a Class

To define a new class, you could use zk.$extends(Class, Map, Map).


com.foo.Location = zk.$extends(zk.Object, {
 x: 0,
 y: 0,
 distance: function (loc) {
  return Math.sqrt(Math.pow(this.x - loc.x, 2) + Math.pow(this.y - loc.y, 2));
 find: function (name) {
  if (name == 'ZK')
   return new com.foo.Location(10, 10);
  throw 'unknown: "+name;

The first argument of zk.$extends(Class, Map, Map) is the base class to extend from. In this case, we extend from zk.Object. The second argument is the (non-static) members of the class. In this case, we define two data members (x and y) and one method (distance).

The third argument defines the static members. In this case we define a static method (find). The third argument is optional. If omitted, it means no static members at all.

Unlike Java, the returned object is the class you defined. You can access it directly, such as o.$instanceof(zk.Widget). In addition, the class object, unlike Java, is not an instance of another class. See more Class.

Access Methods of Superclass

To access the superclass's method, you have to use Object.$super(String) or Object.$supers(String, Array).

com.foo.ShiftLocation = zk.$extends(com.foo.Location, {
 distance: function (loc) {
   if (loc == null) return 0;
   return this.$super('distance', loc);

As shown above, $super is a method (inherited from Object) to invoke a method defined in the superclass. The first argument is the method name to invoke, and the rest of the arguments are what to pass to the superclass's method.

Remember that JavaScript doesn't provide method overloading, so there is only one method called distance per class, no matter what signature it might have. So, it is safer (and easier) to pass whatever arguments that it might have to the superclass. It can be done by the use of $supers.

distance: function (loc) {
 if (loc == null) return 0;
 return this.$supers('distance', arguments); //pass whatever arguments the caller applied


Unlike Java, the constructor is always called Object.$init(), and it won't invoke the superclass's constructor automatically.

com.foo.Location = zk.$extends(zk.Object, {
 $init: function (x, y) {
  this.x = x;
  this.y = y;

Because the superclass's constructor won't be invoked automatically, you have to invoke it manually as follows.

com.foo.ShiftLocation = zk.$extends(com.foo.Location, {
 $init: function (x, y, delta) {
  this.$super('$init', x + delta, y + delta);

Class Metainfo

The class metainfo is available in the class object, which is returned from zk.$extends(Class, Map, Map). With the class object, you can access the static members, examine the class hierarchy and so on.

A class is an instance of Class.


To test if an object is an instance of a class, use Object.$instanceof(Class), or Object.isInstance(Object).

if (f.$instanceof(com.foo.Location)) {
if (com.foo.Location.isInstance(f)) { //the same as above


Each object has a data member called Object.$class, that refers to the class it was instantiated from.

var foo = new com.foo.Location();
zk.log(foo.$class == com.foo.Location); //true

Unlike Java, you can access all static members by the use of the class, including the derived class.

MyClass = zk.$extends(zk.Object, {}, {
 static0: function () {}
MyDerive = zk.$extends(zk.MyClass, {}, {
 static1: function () {}
MyDerive.static0(); //OK (MyClass.static0)
MyDerive.static1(); //OK

However, you cannot access static members via the object.

var md = new MyDerive();
md.static0(); //Fail
md.static1(); //Fail
md.$class.static0(); //OK
MyDerive.static0(); //OK

isInstance and isAssignableFrom

In addition to static members, each class has two important methods, Object.isInstance(Object) and Object.isAssignableFrom(Class).

zk.log(com.foo.Location.isAssignableFrom(com.foo.ShiftLocation)); //true
zk.log(com.foo.Location.isInstance(foo)); //true

Naming Conventions

Private and Protected Members

There is no protected or private concept in JavaScript. We suggest to prefix a member with '_' to indicate that it is private or package, and postfix a member with '_' to indicate protected. Notice it doesn't prevent the user to call but it helps users not to call something he should not.

MyClass = zk.$extends(zk.Object, {
 _data: 23, //private data
 check_: function () { //a protected method
 show: function () { //a public method

Getter and Setter

Some JavaScript utilities the number of arguments to decide whether it is a getter or a setter.

location: function (value) { //not recommended
 if (arguments.length) this.location = value;
 else return value;

However, it is too easy to get confused (at least, with Java's signature) as the program becomes sophisticated. So it is suggested to follow Java's convention (though JavaScript file is slightly bigger):

getLocation: function () {
 return this._location;
setLocation: function (value) {
 this._location = value;

In addition, ZK provides a simple way to declare getter and setters by enclosing them with a special name $define. For example,

$define: {
    location: null,
    label: function (val) {

which defines four methods: getLocation, setLocation, getLabel and setLabel. In addition, setLabel() will invoke the specified function when it is called. For more information, please refer to zk.$extends(Class, Map, Map).

However, if a property is read-only, you can still declare it without get:

distance: function (loc) {
  return Math.sqrt(Math.pow(this.x - loc.x, 2) + Math.pow(this.y - loc.y, 2));

Furthermore, if a property is read-only and not dynamic, you can allow users to access it directly:

if (widget.type == 'zul.wgt.Div') {

Beyond Object Oriented Programming

JavaScript itself is a dynamic language. You can add a member dynamically.

Add a Method Dynamically

To add a method to all instances of a given class, add the method to prototype:

foo.MyClass = zk.$extends(zk.Object, {

foo.MyClass.prototype.myfunc = function (arg) {
 this.something = arg;

To add a method to a particular instance:

var o = new foo.MyClass();
o.myfunc = function (arg) {

To add a static method:

foo.Myclass.myfunc = function () {


Not interface supported, but it can be 'simulated' by the use of the function name. For example, if an interface is assumed to have two methods: f and g, the implementation can just requires by invoking them, and any object that with these two methods can be passed to it.


  • You have to specify this explicitly. Remember it is JavaScript, so the default object is window if you don't.
$init: function () {
 $super('$init'); //Wrong! It is equivalent to window.$super('$init')
  • $init won't invoke the superclass's $init automatically. You have to invoke it manually. On the other hand, you can, unlike Java, do whatever you want before calling the superclass's $init.
$init: function (widget) {
 //codes are allowed here
 this.$super('$init', widget);
 //more codes if you want
  • Data member defined in the second argument of zk.$extends(Class, Map, Map) are initialized only once. For example, an empty array is assigned to the definition of MyClass when the class is defined in the following example.
MyClass = zk.$extends(zk.Object, {
 data: []

It means that all instances of MyClass will share the same copy of this array. For example,

var a = new MyClass(), b = new MyClass();
zk.log(b.data.length); //it becomes 1 since a.data and b.data is actually the same

Thus, to assign mutable objects, such as arrays and maps ({}), it is better to assign in the constructor.

MyClass = zk.$extends(zk.Object, {
 $init: function () {
  this.data = []; //it is called every time an instance is instantiated

Version History

Last Update : 2022/01/19

Version Date Content

Last Update : 2022/01/19

Copyright © Potix Corporation. This article is licensed under GNU Free Documentation License.