Calculate a sum column and save property

Hi I'm new to javascript in ARAS and unfamiliar with the syntax. I have created a new item type called 'purchase order lines' containing some information of manufacturer parts and some additional information. The information is entered manually by the user. It's quantity, price and price_per_unit and should result in a separate column called value as the total value = quanity * (price / price_per_unit). My PO line tab can contain more than one item and the whole column should be calculated whenever there is a change in one of the columns (quantity, price or price_per_unit) and the value property should be updated. I found some tipps in the forum but nothing that works for me. Also I didn't find the OnChange event in the client events tab of the item type. What am I doing wrong? Thanks for the support. Chris