javascript typeof item === “undefined” 从不返回 true

Question

In the following code sample, typeof item === "undefined" never returns true. i am trying to get an attribute from XML, if the attribute is not there in the XML it returns "undefined", but i am not able to capture whether it has returned undefined or not, firebug shows "typeof item" as an "object"

var item;
var itemIDs = {};
if (items.status == 200)
{   
    var rows = items.responseXML.getElementsByTagName('z:row');
    for(i=0;i<rows.length;i++)
    { 
        //rows[i].attr(attribute);
        item = rows[i].getAttribute(attribute);
        if(typeof item === "undefined")
        {
            continue;

        }
        else
        {
            item = item.match(/[^\d+;#][\w\W]+/);
            itemIDs[item] = 1 ;
        }

    }
}
else
{
     alert('There was an error: ' + items.statusText);
}

return itemIDs;

Edit: I changed the condition to if(item == undefined), The code now works as expected now

Edit2: Double checked it, item variable was never null, it was "undefined"

Answer 1

getAttribute returns an object (valid object or a null object). So the check (typeof item === "undefined") is not correct. It should be (item === null) .

Answer 2

Some browser's implementation of getAttribute may return an empty string if the attribute doesn't exist. You could test for both null and "" , or alternatively use hasAttribute .

if(rows[i].hasAttribute(attribute))
{
    // do stuff...
}

https://developer.mozilla.org/en/DOM/element.getAttribute

Answer 3

It's because typeof null === 'object' (contrary to the common sense). You should check if getAttrbiute 's return value equals null .

item = rows[i].getAttribute(attribute);
if (item == null) { /* ... */ }

Answer 4

typeof null is "object" ... this is what getAttribute seems to return when the attribute is missing. See documentation of element.getAttribute , specifically the notes section. It is suggested that you can use hasAttribute .

Answer 5

try this:

if (!item)
{
    continue;
}
else
{
    item = item.match(/[^\d+;#][\w\W]+/);
    itemIDs[item] = 1 ;
}

this proofs if the item is null or undefined. is this true, it continues the loop.

Answer 6

getAttribute : return type object

---

you should compare return value to null