JavaScript classes were introduced in ECMAScript 2015(ES6). They are primarily syntactical sugar over JavaScript’s existing prototype based inheritance.

class Rectangle {
    constructor(width, height) {
        this.width = width;
        this.height = height;
    }

    area() {
        return (this.width * this.height);
    }
}

The constructor method is a special method for creating and initializing an object created with a class.

A constructor can use the super keyword to call the constructor of the super class.

Sub classes can be created with extends keyword.

class Square extends Rectangle {
    constructor(width, height) {
        super(width, height);
    }

    area() {
        let result = super.area();
        console.log("Area of square is ", result);
    }
}

References