How to check if a value is null or undefined in JavaScript (ES2020) [duplicate]

Question

Is there a shorter way to checking if a value in JavaScript is null or undefined than this using new ES10 features like Nullish Coalescing?

(value !== null && value !== undefined)

There were other questions like

But they all use either loose equality (==), use jQuery or return false even if the value is "", [] or 0.

Does it prevent this with eslint? The eqeqeq ESLint rule has the smart option for allowing exactly this type of usage: eslint.org/docs/rules/eqeqeq — loganfsmyth
– loganfsmyth, Commented Feb 5, 2021 at 3:16
@jcalz (value ?? null) !== null might work but is totally weird. I would not recommend to write code like that, it's just a source of confusion. value == null is short, precise, well-known and idiomatic. Nothing about that changed with ES2020. — Bergi
– Bergi, Commented Feb 5, 2021 at 4:52

Pushkin · Accepted Answer · 2021-02-05 03:43:39Z

2

eslint will not throw an error for value == null if smart option is enabled

For project that doesn't have smart eqeqeq rule in eslint config,

There's a shorter way of doing it with Nullish Coalescing.

(value ?? null) !== null

let value

// undefined
console.log((value ?? null) !== null)

// null
value = null
console.log((value ?? null) !== null)

// ''
value = ''
console.log((value ?? null) !== null)

// 0
value = 0
console.log((value ?? null) !== null)

// []
value = []
console.log((value ?? null) !== null)

edited Feb 5, 2021 at 3:43

answered Feb 5, 2021 at 3:23

Pushkin

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2 Comments

jcalz Over a year ago

No, (value ?? false) !== false fails when value === false. If the question were "can we check if a value is null, undefined, or false?" then sure.

Pushkin Over a year ago

Yes, false shouldn't be included. Thanks

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