Populate username field with First name and Last name

popusername.info

name = Populate username
description = Populates username field
php = 5.1
core = 7.x

popusername.js

(function ($) {
  Drupal.behaviors.popusername= {
    attach: function (context, settings) {
      $('#edit-field-first-name-und-0-value, #edit-field-last-name-und-0-value', context).keyup(function () {
        $('#edit-name').val($('#edit-field-first-name-und-0-value').val() + $('#edit-field-last-name-und-0-value').val());
      });
    }
  };
})(jQuery);

popusername.module

<?php
/**
 * @file popusername.module
 * TODO: Enter file description here.
 */

/**
 * Implements hook_form_alter().
 */
function popusername_form_user_register_form_alter(&$form, &$form_state, $form_id) {
  $form['#attached']['js'] = array( drupal_get_path('module', 'popusername') . '/popusername.js');
}