var offset = 10;

​

function setup() {

  createCanvas(240, 120);

  x = width / 2;

}

​

function draw() {

  background(220);

  if (mouseX > x) {

    x += 0.5;

    offset = -10;

  }

  if (mouseX < x) {

    x -= 0.5;

    offset = 10;

  }

  line(x, 0, x, height);

  // draw arrow left/right depending on "offset" value

  line(mouseX, mouseY, mouseX + offset, mouseY - 10);

  line(mouseX, mouseY, mouseX + offset, mouseY + 10);

  line(mouseX, mouseY, mouseX + offset*3, mouseY);

}