Javascript on load

Greetings
I have a table called stock.

Int - ID

Item - varchar

Stock_on_hand - decimal

Required - decimal
When a user requires stock, he/she can choose how many they require whilst seeing Stock_on_hand.

The user should not be able request more than what is currently in stock "Stock_on_hand".

I'm looking at "Calculate total on the fly" but cannot seem to apply it, when I insert a digit it just disappears again.
Below is my Javascript on load

var ctrlRequired = Runner.getControl(pageid, 'Required');

var ctrlStock_on_hand = Runner.getControl(pageid, 'Stock_on_hand');

function func() {

ctrlRequired.setValue(Number(ctrlRequired.getValue()) < Number(ctrlStock_on_hand.getValue()));

};

ctrlRequired.on('keyup', func);
Perhaps someone can guide me into the right direction.
Thanks

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