Typescript Can't implement generic function interface

Question

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:

Edit : Using Typescript 3.0.3

Answer 1

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

Answer 2

So here is what I changed to make it work

ITokenMapper.ts -> IMapper.ts renamed and updated

 export type IMapper<T> = (apiResult: any) => T; 

tokenMapper.ts -> abpTokenMapper.ts renamed and updated

 import { IMapper } from "../../utils/IMapper"; import { IToken } from "../interfaces/IToken"; export const abpTokenMapper: IMapper<IToken> = (apiResult: any) => { if (apiResult.accessToken && apiResult.expireInSeconds) { return { token: apiResult.accessToken as string, expires: apiResult.expireInSeconds as number } } throw new Error('Unable to parse token'); };