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typescripttypesinterfacemapped-types
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Reputation: 21497

Typescript: Mapped types with Interface

When trying to use mapped types with interface, i get a weird error - which makes me think its not possible to use them together at all..

See identical type and interface declarations:

type AllWorks<T> = {
    [K in keyof T]: T[K];
}

interface DoesNotWork<T> {
    [K in keyof T]: T[K];
}

While first one works as expected, second one gives the TS error:

[ts] A computed property name must be of type 'string', 'number', 'symbol', or 'any'.
[ts] Member '[K in keyof' implicitly has an 'any' type.
[ts] Cannot find name 'keyof'.

So my question is - is it even possible to map over interfaces ? if yes - then how ?

Upvotes: 8

Views: 8817

Answers (2)

Jose Greinch
Jose Greinch

Reputation: 458

Interfaces are not built with such a purpose. You should use Type there instead.

The Typescript compiler is telling you that you are using the wrong tool for the job :)

The most complex thing an interface can do is extend otherwise use mapped types as a general rule.

I've built an article here about mapped types if you want to dig deeper.

Upvotes: -1

Behrooz
Behrooz

Reputation: 2371

So far in TypeScript (as of version 2.7.2), union type signatures in interfaces are not allowed and instead mapped types should be used (as you correctly have).

Docs.

Upvotes: 8

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