Strange object is possibly "null" error in TypeScript

Question

I have this simple function that checks if an unknown value looks like a Date object:

function looksLikeDate(obj: unknown): obj is { year: unknown; month: unknown; day: unknown } {
    return (
        obj !== null && typeof obj === "object" && "year" in obj && "month" in obj && "day" in obj
    );
}

But I get the following error for the "year" in obj part of the code:

Object is possibly 'null'. (2531)

When I switch obj !== null and typeof obj === "object" the error goes away: TS Playground Link

Isn't this strange? Can anybody explain this to me?

Does this answer your question? Typescript property type guards on unknown — captain-yossarian from Ukraine
– captain-yossarian from Ukraine, Commented Oct 26, 2021 at 9:18
@captain-yossarian: That doesn't explain that typeof null returns 'object', which is the problem here. — Cerbrus
– Cerbrus, Commented Oct 26, 2021 at 9:20
Is there a reason your type-guard doesn't ensure that year, month, and day are typed as number? If they can be string-or-number then you should use day: string | number. — Dai
– Dai, Commented Oct 26, 2021 at 9:23
This is because obj !== null does not narrow the obj to the object type. After this guard, obj is still unknown. TS does not support negation types, hence it is impossible to express unknown | not null. Whereas, typeof obj === object narrows the obj to specific union object | null. After that you are allowed to extract null with another one typeguard — captain-yossarian from Ukraine
– captain-yossarian from Ukraine, Commented Oct 26, 2021 at 9:28

Cerbrus · Accepted Answer · 2021-10-26 09:26:41Z

2

typeof null === 'object'

If you do the typeof check after the null check, the obj can be null, even though you checked for null before, but the TS compiler is a little naïve.

Those type guards are a little fragile in that sense, as in your example, typeof obj === 'object' changed the type from "not null" to object | null

edited Oct 26, 2021 at 9:26

answered Oct 26, 2021 at 9:19

Cerbrus

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4 Comments

Cerbrus Over a year ago

Yea, it was, @KrisztiánBalla :D

Cerbrus Over a year ago

On that first line? That's intentional. The type of null is 'object'.

Krisztián Balla Over a year ago

Thanks for the explanation. This is not how this should work imho. (A && B) and (B && A) should have the same result.

Cerbrus Over a year ago

Each subsequent type guard changes the type of the value, after it. Those type guards don't take earlier type guards into consideration. typeof <something> === 'object' will always result in that something being typed object | null, which is why you need the null check after it.

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