Define Methods in ES6 Class

class Rectangle {

    constructor(name, a, b) {
        this.name = name;
        this.a = a;
        this.b = b;
    }

    toString() {
        return 'name: ' + this.name + '\na: ' + this.a + '\nb: ' + this.b;
    }

    area() {
        return this.a * this.b;
    }

    perimeter() {
        return (this.a + this.b) * 2;
    }

}

let rectangle = new Rectangle('Rectangle 1', 5, 20);
console.log(rectangle.toString());
console.log('area: ' + rectangle.area());
console.log('perimeter: ' + rectangle.perimeter());

Output

name: Rectangle 1
a: 5
b: 20 
area: 100 
perimeter: 50