Classes

Classes

Are a special type of function that act as a template for object creation.

• Classes dont suffer hoisting.

• They are similar to Constructor Functions.

// Class Declaration
class SomeClass {

}

// Class Expression
const SomeClass = class {

}

They have 3 different member types:

• constructor

• prototype methods

• static methods

constructor

Is invoked at the moment of the class instanciation, and serves to initialize a object.

class Square {
    constructor (side){
        this.side = side;
    }
}

const obj = new Square(4);

prototype methods

They depend on a created instance to be invoked.

class Square {
    ...

    // Prototype method
    toString() {
        return `side: ${this.side}`;
    }
}

const square = new Square(4);

square.toString();

static methods

Dont depend in created instances.

class Square {
    ...

    static fromArea(area) {
        return new Square(Math.sqrt(area));
    }
}

const square = Square.fromArea(16);

extends

Used to create a ``class` hierarchy.

When a class extends a another class, it must call super() before accessing this in the constructor.

class Shape {
    // Notice that the super class can call methods from sub-classes that extends him
    toString() {
        return this.calculateArea();
    }
}

class Square extends Shape{
    constructor (side) {
        super();
        this.side = side;
    }

    calculateArea () {
        return Math.pow(this.side, 2);
    }

    toString() {
        return `side: ${this.side}  area: ${super.toString()}`;
    }
}

class Circle extends Shape{
    constructor (radius) {
        super();
        this.radius = radius;
    }

    calculateArea () {
        return Math.PI * Math.pow(this.side, 2);
    }

    toString() {
        return `radius: ${this.radius}  area: ${super.toString()}`;
    }
}