var y = 350; 

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(20);

  if (keyIsDown(RIGHT_ARROW)) {

    x = x + 3;   

  }

  if (keyIsDown(LEFT_ARROW)) {

    x = x - 3;   

  }

  ellipse(x,y,40,40); 

}