var x; 

​

var vx = 4; 

​

function setup() {

  createCanvas(400, 400);

  x = random(0,400); 

}

​

function draw() {

  background(220);

  ellipse(x, y, 20,20);

  x = x + vx;

  if ( x < 0 || x > 400) {

    vx = -vx;

    console.log(vx);

  }

}