Javascript - 未捕获的TypeError：无法读取未定义的属性“值”

Question

I am currently converting my friends website, as it's not "mobile friendly" by changing his current frontend into a bootstrap front end.

I've been working on a test site which is just a sub-domain on the same server.

This is the old website: http://bit.ly/1hurTNB

This is the new website: http://bit.ly/1hus0IV

But I've encountered a problem with the shopping cart page.

The shopping cart no longer recalculates its total when I press the recalculate button. It works fine on the old website. I dont know what I've changed to break it?

I've debugged the JavaScript using Chrome Dev Tools (F12) and line no 150 of order.php has this error:

Uncaught TypeError: Cannot read property 'value' of undefined 

And this is the line of offending code:

if (document.forms[1].elements["qty" + count].value == "" ||
    document.forms[1].elements["qty" + count].value == 0) {
        document.forms[1].elements["qty" + count].value = 0;
        showInCart = false;
}

I don't understand why I am getting this error? Googling around gives vague answers. Which is why I am here on StackOverflow.

BTW if you want to recreate my problem you'll need to:

1. go to the homepage

2. choose a product category from the grid of pictures e.g. "get costume ideas..."

3. add an item to the basket.

4. once the page posts to Order.php Change its Quantity.

5. click the recalculate button.

Why is the Javascript no longer picking up the values from the quantity fields???

Any help is greatly appreciated.

Here is the function in its entirety:

<script type="text/javascript">
    function recalculateTotal(){        
        var lineTotal = 0;
        var subTotal = 0;
        var total = 0;
        var hiddenStuff = "";
        var count;
        var i;
        var orderURL="register.php?orderDetails=";

        items = new Array(<?=$itemList?>);                  

        for (i in items){
            var cNum = 0;
            var pNum = 0;
            var qNum = 0;

            count = items[i];

            cNum = (count * 4) + 1;

            var showInCart = true;                  

            // the next line is broken!
            if (document.forms[1].elements["qty" + count].value == "" || document.forms[1].elements["qty" + count].value == 0){
                document.forms[1].elements["qty" + count].value = 0;
                showInCart = false;
            }           

            lineTotal = document.forms[1].elements["qty" + count].value * document.forms[1].elements["price" + count].value;                
            document.getElementById("lTotal" + count).innerHTML = formatCurrency(lineTotal);
            subTotal += lineTotal;

            if (hiddenStuff == ""){
                hiddenStuff = hiddenStuff + showInCart + ":" + document.forms[1].elements["productId" + count].value + ':' + document.forms[1].elements["qty" + count].value;
            }else{
                hiddenStuff = hiddenStuff + ":" + showInCart + ":" + document.forms[1].elements["productId" + count].value + ':' + document.forms[1].elements["qty" + count].value;
            }

        }           

        document.getElementById("subTotal").innerHTML = formatCurrency(subTotal);
        for (var j in delivery_prices){
            if (subTotal >= delivery_prices[j].total_amount_start && subTotal <= delivery_prices[j].total_amount_end){
                document.getElementById('delivery').innerHTML = delivery_prices[j].delivery_price;
                total = subTotal + delivery_prices[j].delivery_price;
            }
        }
        document.getElementById("total").innerHTML = formatCurrency(total);
        document.forms[1].elements["orderDetails"].value = hiddenStuff;
        orderURL = orderURL + hiddenStuff;
        var myrequest = $.ajax({
                url: orderURL,
                type: "get",
            });
        myrequest.done(function (response){
            updateBasket();
        });

    }
</script>

Thanks very much.

Answer 1

This error arises when the elements are not either declared or not loaded on the DOM. You can do these things to solve this problem :

1.Wrap every thing inside a function and attach it to the window.onload. Like this :

<script type = "text/javascript">
function foo(){
.....//Your JavaScript code here
}

window.onload = foo;
</script>

1. Check whether all the elements are present in the HTML page or not.

2. I am not sure that this is the problem, but I think its worth mentioning. Change the line no.150 to :

if (document.forms[0].elements["qty" + count].value == "" ||
    document.forms[0].elements["qty" + count].value == 0) {
    document.forms[0].elements["qty" + count].value = 0;
    showInCart = false;
}

In the above code snippet I've changed the occurrence of 1 with 0. Because JavaScript starts counting from 0 not 1. So if you want to access the content of first form then use this point otherwise ignore this point.