How to use JS in multi textbox

Here is my javascript code. I want to use this code multi time in multi textbox can anyone help me?

<--<script type='text/javascript'>
    function totalise()
    {    
        var quantity   = window.document.getElementById('quantity').value;
         var price   = window.document.getElementById('price').value;
        var result = tprice;
        result.value = price * quantity;
    }
</script>-->