var directionRight = true;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(0);

  fill(200, 0, 100);

  rect(rectX, 200, 50, 50);

  // Move the rectangle 

  if (directionRight){

    rectX += 2;

  }

  else{

    rectX -= 2;

  }

  // When the rectangle reaches the edge of the canvas,

  // flip the direction boolean 

  if (rectX > width-50 || rectX < 0 ){

    directionRight = !directionRight;

  }

  // For debugging

  //console.log(rectX);

}