sample.php

/**
 * Implements hook_js_alter().
 */
function THEME_NAME_js_alter(&$javascript) {
  foreach ($javascript as $key => &$file) {
    if ($file['type'] === 'file') {
      $filename_min = preg_replace('/.js$/i', '.min.js', $file['data']);
      if (file_exists($filename_min)) {
        $file['data'] = $filename_min;
      }
    }
  }
}