jquery trigger keyup with letter code example

Example: jquery to copy two input fields into one with a space between

jQuery(function($) {
  var $firstname = $('input[name="firstname"]');
  var $lastname = $('input[name="lastname"]');
  var $fullname = $('input[name="fullname"]');

  $firstname.add($lastname).keyup(function() {
    $fullname.val($firstname.val() + ' ' + $lastname.val());
  });
});