7 – Inline Javascript throws an error on drupal_add_js() within an AJAX callback

I am trying to get JavaScript to measure a text field and auto submit at a character limit for an Ajax callback to be validated against a Luhn Algorithm and errors out on failure to validate.

I found this code in Firing ajax_callback on textfield after a minimum number of character.

// Trigger ajax event on myitem only after 5th character typed.
$('.myitem-class').on('keyup', function() {
  if ($(this).val().length == 5) {

The JavaScript can allegedly be put inline in a wrapper like this to be used in an Ajax callback in a form. See drupal_add_js within AJAX callback.

function my_module_form($form, &$form_state) {
  drupal_add_js('Drupal.ajax.prototype.commands.myFunction = function(ajax, response, status) {
    // …

  return $form;

I got to run replacing function(ajax, response, status) with function($, Drupal).

When I put in the code, it throws a PHP error at ).on(. I wasn’t able to get the inline wrapper recommended in Converting 6.x modules to 7.x / JavaScript should be compatible with libraries other than jQuery to work because it fails seeing the $ as a variable.

(function ($) {
  // (…)

Right now, I am stopped by syntax errors.

Does anyone know how to get this JavaScript code work?