class Ball
{
    constructor(x, y, radius, color)
    {   
        //in this class I am a ball and my name is "this."
        this.x = x;
        this.y = y;
        this.radius = radius;
        this.color = color;
    }   
}
undefined
let ball = new Ball(20, 30)
undefined
let ball = new Ball(20, 30, 20, "red");
Uncaught SyntaxError: redeclaration of let ball
    <anonymous> debugger eval code:1
debugger eval code:1:1
ball = new Ball(20, 30, 20, "red");
Object { x: 20, y: 30, radius: 20, color: "red" }