Customise the response on verification failure for a jwt Strategy NestJs - authentication

Question

I successfully implemented a jwt strategy for authentication using nestJs.
Below is the code for the jwt strategy
import { ServerResponse } from './../helpers/serverResponse.helper';
import { Injectable, UnauthorizedException, HttpStatus } from '#nestjs/common';
import { PassportStrategy } from '#nestjs/passport';
import { ExtractJwt, Strategy } from 'passport-jwt';
import { config as env } from 'dotenv';
import { Bugsnag } from '../helpers/bugsnag.helper';
env();
#Injectable()
export class JwtStrategy extends PassportStrategy(Strategy, 'jwt') {
constructor(
private readonly logger: Bugsnag,
) {
super({
jwtFromRequest: ExtractJwt.fromAuthHeaderAsBearerToken(),
secretOrKey: process.env.JWT_SECRET_KEY,
passReqToCallback: true,
});
}
async validate(payload, done: Function) {
try {
const validClaims = await this.authService.verifyTokenClaims(payload);
if (!validClaims)
return done(new UnauthorizedException('invalid token claims'), false);
done(null, payload);
} catch (err) {
this.logger.notify(err);
return ServerResponse.throwError({
success: false,
status: HttpStatus.INTERNAL_SERVER_ERROR,
message: 'JwtStrategy class, validate function',
errors: [err],
});
}
}
}
I saw here that the validate function will be called only when a valid token was provided in the request headers and I'm okay with that. However, I would like to know if it is possible to customize the response object which is sent in that case (invalid token provided).
If yes, how do I do that ?

Answer 1

You can use a exception filter to catch UnauthorizedExceptions and modify the response there if you'd like. The other option would be extending the AuthGuard('jwt') mixin class and adding in some logic around a try/catch for the super.canActivate(context), then in the error read what the reason is and throw a specific UnauthorizedException with your custom message

Answer 2

You can use the AuthGuard('jwt')'s handleRequest method to throw any exception on JWT Validation failure.
#Injectable()
export class JwtAuthGuard extends AuthGuard('jwt') {
handleRequest(err: any, user: any, info: any, context: any, status: any) {
if (info instanceof JsonWebTokenError) {
throw new UnauthorizedException('Invalid Token!');
}
return super.handleRequest(err, user, info, context, status);
}
}
JsonWebTokenError comes from jsonwebtoken library, which is used internally by passport.