best way to check if something is null or undefined

Question

I was wondering, what's the best way to check if something is null or undefined in JSON arrays or objects in general? For years i've been using

if (value !== null && value !== undefined) {
   // Value isn't null or undefined
}

You can use `if(!value)` but it will be true for `''` and `0` as well. — Konrad, Sep 10 '22 at 11:19
`value != null` does the same thing as your if-statement, but it's not necessarily better than what you have. — Nick Parsons, Sep 10 '22 at 11:22
you can use the loose equality operator against null if(value == null) to check for both null and undefined — overflowed, Sep 10 '22 at 11:23
There is no such thing as JSON *array*. In a JSON stringified object, `undefined` will be converted to `null` — KooiInc, Sep 10 '22 at 11:28
depending on the logic of the `if` block, you might want to take a look and consider using the nullish coalescing operator `??` or the nullish assignment `??=` — PA., Sep 10 '22 at 11:28
Maybe [this](https://stackblitz.com/edit/js-4f2faa?file=index.js) helps — KooiInc, Sep 10 '22 at 11:36
@ikhvjs - Reliability isn't a personal preference. The OP's way is reliable, and so is `value != null`. The only issue with the second way is whether future programmers will know the rules well enough to know that's the same thing. — T.J. Crowder, Sep 10 '22 at 11:36
@KonradLinkowski yes, I already fixed this in my answer, and forgot about comment above, thanx — tarkh, Sep 10 '22 at 12:58

Xion 14 · Answer 1 · 2023-09-01T11:19:21.383

You can just use the loose equality operator ==.
It will treat null and undefined as if they were the same.

if (value != null) {
  /* do something with the value if it isn't null AND undefined */
}

if (value == null) {
  /* do something with the value if it is null OR undefined */
}

// Which is the same as this one with the strict equality operator

if (value !== null && value !== undefined) {
  /* do something with the value if it isn't null AND undefined */
}

if (value === null || value === undefined) {
  /* do something with the value if it is null OR undefined */
}

tarkh · Answer 2 · 2022-09-10T14:44:26.440

-1

IMHO using typeof for undefined is the safest way, because it does not requires variable to be declared at all. Then you can add value !== null, so if value declared, then check that it is not Null. Of course it depends on your project details, code style etc., and there is no such thing as "best"... But if your environment expects situation, where variables could be not declared or you have some dynamic function generation, keep in mind, that typeof value !== "undefined" will handle undeclared variables.

if (typeof value !== "undefined" && value !== null) {
  console.log("exist and not Null");
} else {
  console.log("not exist or Null");
}

Below is the example, which will throw error if not using typeof:

if (value !== undefined && value !== null) {
  console.log("exist and not Null");
} else {
  console.log("not exist or Null");
}

edited Sep 10 '22 at 14:44

answered Sep 10 '22 at 11:35

tarkh

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@T.J.Crowder, ah, sorry) I mean exactly what you wrote, but lost focus. Fixed. About answers - I agree, stackoverflow needs some feature with merging questions. Anyway thanx for pointing this out. – tarkh Sep 10 '22 at 12:10
I would expect to get an error if I forgot to declare a variable – Konrad Sep 10 '22 at 13:30
so... which one is the best? – sda dsasdsa Sep 10 '22 at 14:13
@sdadsasdsa, there is no such thing as "best"... But if your environment expects situation, where variables could be not declared or you have some dynamic function generation, keep in mind, that `typeof value !== "undefined"` will handle undeclared variables. – tarkh Sep 10 '22 at 14:45

best way to check if something is null or undefined

2 Answers2