Detecting if object is null in Javascript - why doesn't this work?

Question

I'm trying to figure out why I get this error from some visitors via my JS error console tracker:

Uncaught TypeError: Cannot read property 'classList' of null

The code in question is:

                if (typeof item == "object") {
                    item.classList.remove('active');
                    update_specific_marker(item.getAttribute('data-what'),"remove_active_icon");
                }

Shouldn't this bit work:

if (typeof item == "object") {

I'm baffled as to how they get it in the first place - as I can't get any errors to come out from that page (no matter how much I play with it, even in Chrome which is where it seems to come from)

Any suggestions on what I could try?

Have you tried console.log(item) before item.classList.remove('active');? — Beller, Dec 26 '19 at 08:17
If you want to be sure it's not null do `if (item !== null)` if you want to be sure it exists and is not null do `if (typeof item !== 'undefined' && item !== null)` — slebetman, Dec 26 '19 at 08:18
You can use `Object(item) === item` to check if it is an object. This returns false for `null`, strings, numbers etc. — adiga, Dec 26 '19 at 08:19
@adiga `item === null` suffices. In most contexts, `!item` suffices (even in this case: typically no falsy things have `.classList`). — Amadan, Dec 26 '19 at 08:20
@adiga: Ah, yeah. Sorry, did not understand that from what you wrote. — Amadan, Dec 26 '19 at 08:22
@Amadan updated comment. TBF, `Object(item) === item` returns true for functions. So, there is that. — adiga, Dec 26 '19 at 08:23
Thanks everyone - I wasn't aware null was still considered an Object in JS. I'll rweak the code as suggested :) — Andrew Newby, Dec 26 '19 at 09:15

Prithwee Das · Answer 1 · 2019-12-26T10:32:03.090

2

If you do typeof null in a JS console, it will print object. so in your case if item = null then also typeof item == "object" will produce true

what you can do instead is

if(item){
  // do things
}

edited Dec 26 '19 at 10:32

answered Dec 26 '19 at 08:18

Prithwee Das

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score 1 · Accepted Answer · answered Dec 26 '19 at 08:19

For item = null, typeof item == "object" is true, so inside the block you're trying to access the classList property of a null value. You can't do that, hence the error.

To fix it, you're going to need another condition to stop null values from accessing the code.

if (typeof item == "object" && item != null)

Detecting if object is null in Javascript - why doesn't this work?

2 Answers2