What is the 0 define at the end of each function in this code?
Why this code set 0s at the end of the function?

```js
canvas.addEventListener('mousedown', function(e) {
  model.dragging = getCircleForPosition(e.pageX, e.pageY);
}, 0);
canvas.addEventListener('mouseup', function() {
  model.dragging = undefined;
}, 0);
canvas.addEventListener('mousemove', function(e) {
  if (model.dragging) {
    model.dragging.x = e.pageX;
    model.dragging.y = e.pageY;
    redraw();
  }
}, 0);
```