If statement checking not null is being ignored

Question

Below is a portion of code I'm using to get the href or src of an image of known class or id. The console.log() returns null, even though it is in an if statement checking that the attribute later used isn't null. Of course, trying to get href or src of null doesn't end well.

for(var i = 0 ; (i < sitelist[site].img_id.length) && (img === undefined) ; i++)
{
    if(document.getElementById(sitelist[site].img_id[i]) !== undefined)
    {
        if(document.getElementById(sitelist[site].img_id[i]) !== null)
        {
            console.log(document.getElementById(sitelist[site].img_class[i]));
            if(document.getElementById(sitelist[site].img_class[i]).href !== undefined)
            {
                img =   document.getElementById(sitelist[site].img_class[i]).href;
            }
            if(document.getElementById(sitelist[site].img_class[i]).src !== undefined)
            {
                img =   document.getElementById(sitelist[site].img_class[i]).src;
            }
        }
    }
}

Is there a specific way of checking if something is null, or is the problem elsewhere?

@PrasathK [yes there is](http://stackoverflow.com/q/801032/447356). Maybe not the same like in languages such as C or C# but `null` is an object thus it exists. — Shadow The GPT Wizard, May 22 '13 at 12:41
Of course there is null in js, you're completely wrong! http://www.w3schools.com/js/js_datatypes.asp?output=print — Oscar, May 22 '13 at 12:41
Better reference, in my opinion: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/null — Andrew, May 22 '13 at 12:46
ekhm ... what about: var x = null; if(x === null) alert('nulls exists!'); ? — Slawomir Pasko, May 22 '13 at 12:46
@Andrew—best reference is [ECMA-262](http://www.ecma-international.org/ecma-262/5.1/#sec-7.8.1). — RobG, May 22 '13 at 12:54
@Andrew—heaven forbid a programmer should learn to read the language specification. ;-) — RobG, May 22 '13 at 12:58
@Andrew Then use the more readable version: http://es5.github.io/ :) — Ian, May 24 '13 at 05:02

score 2 · Answer 1 · edited May 22 '13 at 12:51

2

You can just try the below code.

Replace

if (document.getElementById(sitelist[site].img_id[i]) !== undefined) {
    if (document.getElementById(sitelist[site].img_id[i]) !== null) {
        // ...
    }
}

with

if (document.getElementById(sitelist[site].img_id[i])) {
    // process here
}

You are checking null and undefined continuously in next statement. You can just try with element only.

edited May 22 '13 at 12:51

Paul S.

64,864
9
122
138

answered May 22 '13 at 12:43

SivaRajini

7,225
21
81
128

http://stackoverflow.com/questions/2559318/how-to-check-for-undefined-or-null-variable-in-javascript – SivaRajini May 22 '13 at 12:54
The problem wasn't on the structure but on the variables used. The test was on img_id and img_class was used. Your answer is perfectly true, but the problem wasn't there. – Caramel Truffle May 22 '13 at 13:13

score 1 · Answer 2 · edited May 22 '13 at 12:43

1

Replace

   id[i]) !== undefined

with

   id[i]) != undefined

in all statements

edited May 22 '13 at 12:43

Prasath K

4,950
7
23
35

answered May 22 '13 at 12:39

PSR

39,804
41
111
151

@CaramelTruffle yes you need to change in all places – PSR May 22 '13 at 12:47
@CaramelTruffle did you tried this by replacing in alla places – PSR May 22 '13 at 12:53
I will remember it, thanks. The source of the problem seems to be somewhere else though. Edit: And yes, I replaced all !== by != – Caramel Truffle May 22 '13 at 12:53

score 1 · Answer 3 · answered May 22 '13 at 12:41

1

To test if the element is found, simply use

var elem = document.getElementById(sitelist[site].img_id[i]);
if (elem) {
   // proceed from there

Note that I also declared a variable. Repeating your document.getElementById(sitelist[site].img_id[i]) makes your code hard to read (and thus debug and maintain).

answered May 22 '13 at 12:41

Denys Séguret

372,613
87
782
758

I agree for the readability part, but isn't it bad concerning the time required to execute the code? – Caramel Truffle May 22 '13 at 12:57
@CaramelTruffle On the contrary : it avoids fetching the element more than once. – Denys Séguret May 22 '13 at 13:01
I will remember it and start using it then. Thanks. – Caramel Truffle May 22 '13 at 13:11

score 0 · Accepted Answer · 2013-05-22T13:07:10.433

Your null and undefined tests are correct, though longer than needed.

Your actual issue is that you're testing for .img_id[i], but then using .img_class[i].

Here's a more concise version of your existing code.

for(var i = 0 ; (i < sitelist[site].img_id.length) && (img === undefined) ; i++) { 

// Here you're testing the `img_id[i]`
    var img_id = document.getElementById(sitelist[site].img_id[i]);

    if(img_id != null) {

// Here you're trying to use a different element
        var img_clss = document.getElementById(sitelist[site].img_class[i]);
        console.log(img_clss);

        if(img_clss.href !== undefined) {
            img =   img_clss.href;
        }
        if(img_clss.src !== undefined) {
            img = img_clss.src;
        }
    }
}

Unless the values are the same, testing for one isn't going to tell you if a different one exists.

I don't know what the intent is, but perhaps you just need to change the element inside the if.

for(var i = 0 ; (i < sitelist[site].img_id.length) && (img === undefined) ; i++) { 
    // Here you're testing the `img_id[i]`
    var img_id = document.getElementById(sitelist[site].img_id[i]);

    if(img_id != null) {
        // Now we're using the same element that we tested!
        console.log(img_id);

        if(img_id.href !== undefined) {
            img = img_id.href;
        }
        if(img_id.src !== undefined) {
            img = img_id.src;
        }
    }
}

Thank you! The real problem was truly that I was checking img_id then using img_class. Shame on me. I will ameliorate its readability and compactness though. — Caramel Truffle, May 22 '13 at 13:10

If statement checking not null is being ignored

4 Answers4