<?php

class CustomTextField extends TextField {
   
   function __construct($name, $title = null, $value = "", $maxLength = null, $form = null){
      
      $this->include_js($name);
      parent::__construct($name, $title, $value, $maxLength, $form);
   }
   
   public static function include_js($name){
      Requirements::javascript('productadmin/testcode/test.js'); // <--- works
   }
   
   function Field() {
      $attributes = array(
         'type' => 'text',
         'class' => 'text' . ($this->extraClass() ? $this->extraClass() : ''),
         'id' => $this->id(),
         'name' => $this->Name(),
         'value' => $this->Value(),
         'tabindex' => $this->getTabIndex(),
         'maxlength' => ($this->maxLength) ? $this->maxLength : null,
         'size' => ($this->maxLength) ? min( $this->maxLength, 30 ) : null
      );
      
      if($this->disabled) $attributes['disabled'] = 'disabled';
      Requirements::javascript('productadmin/testcode/test.js');   // <--- works   
      return $this->createTag('input', $attributes);
   }
}