Autocomplete is a feature for textfields in Drupal. It provides a dropdown list of matching options from the server. It is implemented through AJAX. To know how to add an autocomplete form element in Drupal visit https://www.drupal.org/node/854216

For customizing autocomplete we need to override the Drupal system file "misc/autocomplete.js". This can be achieved ideally in two ways:

• Replace the entire "autocomplete.js" with your customised version

  function MY_MODULE_js_alter(&$javascript) {  $javascript['misc/autocomplete.js']['data'] = drupal_get_path('module', 'MY_MODULE') . '/js/autocomplete.js';}

• Override "Drupal.ACDB.prototype.search" with a custom behaviour in theme script.js or module js file, as long as it's added after /misc/autocomplete.js it will override it.