compute sum dynamically with javascript

[bar1912]

I would like to compute sum of subtotals dynamically but i always get this error:\[code\]document.getElementById("item_subtotal[" + cnt + "]") is null\[/code\]My javascript code: \[code\]function calculateTotalAll(numitems) { var cnt = 1; var totalAmt = 0; while(cnt <= numitems) { totalAmt = parseInt(totalAmt) + parseInt(document.getElementById('item_subtotal[' + cnt + ']').value); cnt++; } document.getElementById('order_total').value = http://stackoverflow.com/questions/3939577/parseInt(totalAmt); }\[/code\]