Using reusable function to check if array instead of Array.isArray in typescript throws error

Question

In typescript, if I use this,

Array.isArray(value) ? [value] : value

...it works fine. But if I use a utility function to check it if is array then Typescript complains with error message Argument of type Item[] | Item[][] is not assignable to type Item | Item[].

const isArray = (value: any) => Array.isArray(value)

isArray(value) ? [value] : value // Throws error

Try const isArray = (value: any): value is any[] => Array.isArray(value) — Orkhan Alikhanov
– Orkhan Alikhanov, Commented Feb 24, 2021 at 9:16
Why not just const { isArray } = Array? Given that the function only does that one thing. Or is that just an example? — Lionel Rowe
– Lionel Rowe, Commented Feb 24, 2021 at 9:19
@LionelRowe That's just my personal preference to have like all the utility functions in one file be it as simple as checking array, string, and so on... 😅 — Axel
– Axel, Commented Feb 24, 2021 at 9:22
In that case yeah, just use const { isArray } = Array. No need to create a new function just to call an existing function. — Lionel Rowe
– Lionel Rowe, Commented Feb 24, 2021 at 9:29

Orkhan Alikhanov · Accepted Answer · 2021-02-24 09:47:01Z

1

Try:

const isArray = (value: any): value is any[] => Array.isArray(value)

You can learn about this more here

edited Feb 24, 2021 at 9:47

answered Feb 24, 2021 at 9:18

Orkhan Alikhanov

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