How to extend function declaration defined by DefinitelyTyped community?

Question

I'm using jsonwebtoken library within TypeScript project. Together with this library, I've imported @types/jsonwebtoken library to provide types. Within this library jsonwebtoken's function verify declared as following:

export function verify(
  token: string, 
  secretOrPublicKey: Secret, 
  options?: VerifyOptions
): object | string;

But I would like to specify which object exactly it returns, not just object | string for example, an object defined by the following interface:

export interface DecodedJwtToken {
  userId: string;
  primaryEmail: string;
}

How can I achieve it within my project? Can it be done without type conversion, i.e.

const decodedToken: DecodedJwtToken = verify(token, JWT_PRIVATE_KEY) as DecodedJwtToken;

Thank you in advance.

Answer 1

What you're looking for is module augmentation:

import { Secret, VerifyOptions } from 'jsonwebtoken';

export interface DecodedJwtToken {
    userId: string;
    primaryEmail: string;
}

declare module 'jsonwebtoken' {
    function verify(token: string, secretOrPublicKey: Secret, options?: VerifyOptions): DecodedJwtToken;
}