CODEWITHSUNDEEP
typescripttypesdecorator
Chen An
Chen An

Reputation: 81

Is there any type inference for method decorator in TypeScript?

Since there is TypedPropertyDescriptor which can be used to define method decorators, is there any way to let the compiler to infer parameter types of decorated methods?

function test(
    target: any,
    propName: string | symbol,
    descriptor: TypedPropertyDescriptor<(x: number) => any>
) {
}

class T {
  @test
  log(n) {  // <-- compiler complains that n has type of implicit any

  }
}

As (x: number) => any was passed into TypedPropertyDescriptor, it implies that all methods decorated by test should be of type (x: number) => any, thus above code should type check.

So TypeScript doesn't support this kind of inference yet, or do I miss something which could make this code type check?

Upvotes: 1

Views: 936

Answers (1)

Fenton
Fenton

Reputation: 250922

There are many cases where TypeScript undertakes contextual typing, but this isn't one.

Add the type annotation to the parameter:

class T {
  @test
  log(n: number) {

  }
}

The type is still checked in relation to the decorator, even though the decorator does not provide contextual type information:

class T {
  @test
  log(n: string) { // ERROR!

  }
}

Upvotes: 4

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