Infer generic argument when extending a Typescript interface

Question

I have an interface which requires a generic variable:

interface Action<T extends string> {
   type: T
}

I am able to extend it when I set the value of the generic:

interface SomeAction extends Action<"something"> {
}

I would like to infer the generic automatically, but I get an error when I try this:

interface SomeAction extends Action {
   type: "something"
}

Error TS2314: Generic type 'Action' requires 1 type argument(s).

I'd like the second example to infer its type automatically. Is this possible?

Eldar · Accepted Answer · 2020-11-01 13:31:48Z

4

Just specify default value for T and it should work:

interface Action<T extends string = string> {}

Eldar

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answered Nov 1, 2020 at 13:06

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3 Comments

@Eldar Are you sure? Can you provide some examples in which such a solution wouldn't work?

Sorry, my bad to make that assumption.

@Eldar It's okay, everyone can make a wrong assumption :)