HTML Form Field Deduction 1

[pastorandy]

I have two form fields and I want to be able to subtract the value from one. What I currently have doesn't work.

I haven't written a javascript function before but this is what I have:

<script language="JavaScript">
function deduct() 
{
document.form1.payment_amount.value = --document.form1.payment_deduction.value;
}
</script>

I was hoping that whatever value was in payment_deduction would get subtracted from payment amount.

I have the two form fields and a button to trigger the function:

<input type="button" name="btnSub" onClick="javascript:deduct();">

Could someone point me in the right direction?

 

[tsuji]

As a start, but you have to make a lot of assumptions on the user's entries.
[tt]document.form1.payment_amount.value-=document.form1.payment_deduction.value;[/tt]

 

[pastorandy]

Works great thanks! )