javascript变量返回未定义状态

Question

I am working with some javscript at the moment, the basic thought process behind it is that, you click a link and that links produces and entry in a basket, I currently logged what gets sent to the basket in my console. When I click a link it triggers the additem() function,

function additem(desc,price,quantity)
    {

        var x = items.length - 1;
        var found = (x == 0);
        var y = new Item(desc,price,parseInt(quantity));
        console.log(y);
        var z = y;

        while((x != 0) && (found == false))
        {
            z = items[x];

            if(z != null)
            {
                if(z.M_desc == desc)
                {
                    found = true;
                }
                else
                {
                    x--;
                }
            }
            else
            {
                x--;
            }
        }

        if(found == false)
        {
            items[++numitems] = y;
        }
        else
        {
            items[x].M_quantity += quantity;
        }

        updatecart();
    }

What I see in the console is the following,

Item { M_desc="Item 2", M_price="10", M_quantity=1}

however when I click into it, I get the following ,

M_desc
    "Item 2"

M_price
    "10"

M_quantity
    undefined

How can M_quantity be 1 and then undefined in another console view?

===== EDIT =====

I think the problem stems form here, my JS code starts with the line.

var items = new Array(2);

now I assume that this creates a new array as when I run console.log(items) straight after it I get [undefined, undefined] .

additem is called on the click of a link using the following code,

HTML

<a class='product' rel='"+category[i].product[j].price+"' href=''>"+ category[i].product[j].description + "</a>

JS

$('.product').live("click",function(e){
    additem($(this).text(), $(this).attr('rel'), 1);
e.preventDefault();
});

This is when the problems arise as items[x] is either always null or undefined. and I not sure why.

Answer 1

Your updatecart function is being called almost simultaneously. items[i].M_quantity = $('input[name="qty"]').val() is the problematic line. The input is null and is replacing your quantity with undefined.