let x = 0;

let speedY=4;

let y=0; 

​

function setup() {  

   createCanvas(400, 400);

}

​

function draw() {  

   background(0);  

   stroke(255);  

   strokeWeight(4);  

   noFill();  

    rectMode(CENTER)

   rect(x, y, 40, 30); 

  //Checking canvas edges

  if(x>width||x<0){

    speedX *=-1;

  } 

   if(y>height||y<0){

     speedY*= -1;

 }

x = x + speedX;

y = y+speedY;

}

​