How to add some JavaScript functions before Ajax begins
"beforeSubmit" handler attached that way will be executed right before Ajax request:

(function ($) {
  Drupal.behaviors.ReplaceWithAnyTextYouWant = {
    attach: function (context, settings) {
      // 'edit-submit' is the element ID where you execute the #ajax.
      Drupal.ajax['edit-submit'].options.beforeSubmit = function(form_values, element_settings, options) {
      // If cancel is clicked, do not continue submitting.
      if (!confirm('Are you sure you want to continue?')) {
          return false;
      }
    }
  };
})(jQuery);


You could perform some pre-submit validation of the user input, and even cancel form submission by returning false

Here is a good example

'edit-submit' - is the element html ID of submit button with '#ajax' settings. 'edit-' prefix is attached to the key of your form array item containing triggering element (submit button in your case).
For $form['my-submit'] = array(...), html ID of the resulting button will: "edit-my-submit".

JS file with code shown above, could be attached to your form in form constructor:

$form['#attached']['js'][] = drupal_get_path('module', 'your_module') . '/js/file.js';