Fastify & NestJS - How to set response headers in interceptor - header

Question

I'm trying to set the response headers in my interceptor, and have had no luck with any method I've found yet. I've tried:
const request = context.switchToHttp().getRequest();
const response = context.switchToHttp().getResponse();
<snippet of code from below>
return next.handle();
request.res.headers['my-header'] = 'xyz'
response.header('my-header', 'xyz')
response.headers['my-header'] = 'xyz'
response.header['my-header'] = 'xyz'
with no luck. The first option says that res is undefined, the second "Cannot read property 'Symbol(fastify.reply.headers)' of undefined", and the others just do nothing.

Answer 1

I have the following working for me with the FastifyAdapter in my main.ts:
HeaderInterceptor
#Injectable()
export class HeaderInterceptor implements NestInterceptor {
intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
return next.handle().pipe(
tap(() => {
const res = context.switchToHttp().getResponse<FastifyReply<ServerResponse>>();
res.header('foo', 'bar');
})
);
}
}
Using .getResponse<FastifyReply<ServerResponse>>() gives us the correct typings to work with.
AppModule
#Module({
imports: [],
controllers: [AppController],
providers: [
AppService,
{
provide: APP_INTERCEPTOR,
useClass: HeaderInterceptor,
},
],
})
export class AppModule {}
Bind the interceptor to the entire server
curl Command
▶ curl http://localhost:3000 -v
* Rebuilt URL to: http://localhost:3000/
* Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 3000 (#0)
> GET / HTTP/1.1
> Host: localhost:3000
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 200 OK
< foo: bar
< content-type: text/plain; charset=utf-8
< content-length: 12
< Date: Thu, 14 May 2020 14:09:22 GMT
< Connection: keep-alive
<
* Connection #0 to host localhost left intact
Hello World!%
As you can see, the response comes back with the header foo: bar meaning the interceptor added what was expected.
Looking at your error, it looks like your second attempt may have actually been response.headers('my-header', 'xyz). Whatever the case, the above is working for me on a nest new application, and on the latest version of Nest's packages.