I wonder there is a difference between...

document.addEventListener('DOMContentLoaded', function() { callback() });

and

document.addEventListener('DOMContentLoaded', callback);

I've seen code like this, and I'm curious as to why they put the function inside an anonymous function.

var on_load = function(f) {
  if (document.body === null)
    document.addEventListener('DOMContentLoaded', function() { f() }, false)
  else
    f()
}