let canvas = document.querySelector("#canvas");
let ctx = canvas.getContext("2d");
let width = canvas.width;
let height = canvas.height;
const BALL_SIZE = 5;
let ballPosition = {x: 20, y: 30};
function draw() {
ctx.fillStyle = "black"; ctx.fillRect(0, 0, width, height);
     ctx.fillStyle = "white";
     ctx.fillRect(ballPosition.x, ballPosition.y, BALL_SIZE, BALL_SIZE);
   }
draw();