var speed = 1 , speed1 = 1;

function setup() {

  createCanvas(800, 400);

}

​

function draw() {

  background(220);

    rect(x,y,100,100);

    x += speed;

    y += speed1;

    if(x+100 >=width || x <= 0)

        speed = -speed;

    if(y+100 >=height || y <= 0)

        speed1 = -speed1;

}