Calculating Totals

HTML

<input type="text" id="item_subtotal[1]" value=""><br>
<input type="text" id="item_subtotal[2]" value=""><br>
<input type="text" id="item_subtotal[3]" value=""><br>
<input type="text" id="item_subtotal[4]" value=""><br>
<input type="text" id="item_subtotal[5]" value=""><br>

Total: <span id="order_total">?</span>

<button onClick="calculateTotal(5)">Update</button>

JavaScript

function calculateTotal(numitems) {
    var totalAmt = 0;

    for (var cnt = 1; cnt <= numitems; cnt++) {
        var subtotal = document.getElementById('item_subtotal[' + cnt + ']');
        if (subtotal.value === null || subtotal.value === '') {
            continue;
        }

        totalAmt += (subtotal.value * 1);
    }

    document.getElementById('order_total').innerHTML = totalAmt;
}