Typescript nested array mistyping issue

Question

I need to pass data with the structure shown below

export const Legend = ({ data }: { data: [string, number][] }) => ....

Why Typescript types test array in the way like in picture (string | number)[][], instead of [string, number][]

Of course, I can overwrite the type, but I do not think this is a good approach

If your question is really "why does Typescript do this?", the answer is because if it inferred [string, number] instead of (string | number)[] for an array containing a string and a number, a lot more people would be annoyed about it. Consider using a different type like {s: string, n: number}, or just write the type annotation. — kaya3
– kaya3, Commented Jul 30, 2022 at 15:11
Please replace/supplement images of code/errors with plaintext versions. — jcalz
– jcalz, Commented Jul 30, 2022 at 20:16

user-id-14900042 · Accepted Answer · 2024-07-17 04:29:08Z

3

No, it is your side issue. [string, number][] is an array whose elements are string and number pair. I assume you'd like to type test more specific than (string | number)[][] (NOTE: If so, it is called XY-problem, which you should avoid).

You may want to use const-assertion, which can be done by postfix expression by as const, like this:

const test = [
    ['C2B', 'D2C', 'PDP', 1],
    ['C2B', 'D2C', 'PDP', 2],
    ['C2B', 'D2C', 'PDP', 4],
    ['C2B', 'D2C', 'PDP', 2],
] as const;

You should change Legend signatures, too:

export const Legend = ({ data }: { data: readonly (readonly [string, string, string, number])[] }) => ...

The test elements are typed an specific type which is subtype of readonly [string, string, string, number]. The outer readonly modifier means entire array is read-only and you cannot change its contents. This is side effect of const-assertion, which does not have opt-out way.

edited Jul 17, 2024 at 4:29

answered Jul 30, 2022 at 14:36

user-id-14900042

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