how to extract JWT when authorize in Nestjs - node.js

Question

My purpose is make a role guard to verify the permission of user. I trying to extract authorization header to get the role information which is include on JWT. I implemented canActivate interface to check role of use but i don't know how to get the role info from JWT to verify it.
export class RolesGuard implements CanActivate {
constructor(private readonly _reflector: Reflector) {
}
canActivate(context: ExecutionContext): boolean {
const roles = this._reflector.get<UserRole[]>(
'roles',
context.getHandler(),
);
if (!roles || roles.length === 0) {
return true;
}
const request = context.switchToHttp().getRequest();
const user: InstanceType<User> = request.headers.role;
// i want to get the role from JWT in here
const hasRole = () => roles.indexOf(user.role) >= 0;
if (user && user.role && hasRole()) {
return true;
}
throw new HttpException(
'You do not have permission (Roles)',
HttpStatus.UNAUTHORIZED,
);
}
}
I tried extends PassportStrategy, but it can't work together with CanActive

Answer 1

One option is to use the JwtService from the JwtModule and use jwtService.decode(myJwt) to get the decoded JWT and get the role from there. The other is to use the built Passport Guard (AuthGuard), extend the functionality, and call super.canActivate(context) before your custom logic. Store he result and immediately check if the user has passport access before continuing with your custom logic.
// the mention of jwt in the AuthGuard is only needed if not working with defaultStrategy
export class RolesGuard extends AuthGuard('jwt') {
constructor(private readonly _reflector: Reflector) {
super()
}
canActivate(context: ExecutionContext): boolean {
const passportActive = super.canActivate(context);
if (!passportActivate) {
throw new HttpException(
'You do not have permission (Roles)',
HttpStatus.UNAUTHORIZED,
);
}
const roles = this._reflector.get<UserRole[]>(
'roles',
context.getHandler(),
);
if (!roles || roles.length === 0) {
return true;
}
const request = context.switchToHttp().getRequest();
// this should come from passport
const user: InstanceType<User> = request.user;
// i want to get the role from JWT in here
const hasRole = () => roles.indexOf(user.role) >= 0;
if (user && user.role && hasRole()) {
return true;
}
throw new HttpException(
'You do not have permission (Roles)',
HttpStatus.UNAUTHORIZED,
);
}
}