The extends keyword is used to define a class as a child of another class. The following example demonstrates inheritance in practice:

class TextBox extends Widget {
    constructor (id, x, y, text) {
        super(id, x, y);
        this.text = text;

We created a new TextBox class that is based on the Widget and adds additional text property.

Note that a base Widget constructor must also be called when a child class instantiated. It must be the very first line of the child constructor implementation.

Here's another example:

class ImageBox extends Widget {
    constructor (id, x, y, width, height) {
        super(id, x, y);
        this.setSize(width, height);

    setSize(width, height) {
        this.width = width;
        this.height = height;

    reset() {
        this.setPosition(0, 0);
        this.setSize(0, 0);

ImageBox also inherits Widget class and adds size-related information alongside position.

Access to both classes is demonstrated with the reset function that calls Widget.setPosition and ImageBox.setSize functions.