update field

hi,

I'm new on suitescript and try to create files for testing purpose, i want to craete a script to update purcahes description for a specific item (which have internal id = '2592')to be the exist description + some text (123 as example). so i upload .js file contains the code bellow:

function updateDescription()
{
   if (nlapiGetFieldValue('internalid')=='2592')
  {
      var Desc = nlapiGetFieldValue('purchasedescription');
      Desc = Desc + '123';
      nlapiSetFieldValue('purchasedescription',Desc);
   }
}

it retrieve the field value and change variable Desc but it didn't set the new value for purchase description, i tried to create client script (pageinit function) and another user event script (beforeload function); but no one set the new value