function add(first, second, callback) {
  console.log(first+second);
  if (callback) { //don't fire callback if it doesnt exist.
    callback();
  }
}

add(2, 3, function(){
    console.log('done');
});

add(4,5, function() {
  console.log('done again');
});

add(4,5);


//Jquery example

$('#myDiv'.click(function (){
  //function will only fire when you click.
}))