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Typescript Can't implement generic function interface

I'm trying to implement a generic function interface and I cannot make it work.

IToken.ts 

export interface IToken {
  token: string;
  expires: number;
}

ITokenMapper.ts

export interface ITokenMapper {
  <T>(apiResult: any): T;
}

tokenMapper.ts

import {ITokenMapper} from "./interfaces/ITokenMapper";
import {IToken} from "./interfaces/IToken";

export const tokenMapper: ITokenMapper = function <IToken>(apiResult: any): IToken {
  if(apiResult.token && apiResult.expires) {
    return {token: apiResult.token as string, expires: apiResult.expires as number}
  }
  throw new Error('Unable to parse token');
};

Here is a screenshot from tokenMapper.ts saying IToken import is unused but I should have a use for it:

tokenMapper.ts

Edit : Using Typescript 3.0.3

Solution

  • I believe you can accomplish your typing with a generic interface ITokenMapper<T>

    interface IToken {
    token: string;
    expires: number;
}

interface ITokenMapper<T> {
    (apiResult: T): T;
}

const tokenMapper: ITokenMapper<IToken> = function (apiResult) {
    if(apiResult.token && apiResult.expires) {
      return { token: apiResult.token as string, expires: apiResult.expires as number};
    }

    throw new Error('Unable to parse token');
};

    From: https://www.typescriptlang.org/docs/handbook/generics.html#generic-types