How to add multiple headers in Angular 5 HttpInterceptor

Edit: Following suggestions in comments I have unified both answers

Overwriting all headers

@Injectable()
export class fwcAPIInterceptor implements HttpInterceptor {
  intercept (req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {

  const authReq = req.clone({
    headers: new HttpHeaders({
      'Content-Type':  'application/json',
      'Authorization': 'my-auth-token'
    })
  });

  console.log('Intercepted HTTP call', authReq);

  return next.handle(authReq);
}

Adding more headers without overwriting (credits to Ketan Patil - see answer in this post)

const authReq = req.clone({
    headers: req.headers.set('Content-Type', 'application/json')
        .set('header2', 'header 2 value')
        .set('header3', 'header 3 value')
});

Since the set method returns headers object every time, you can do this. This way, original headers from the intercepted request will also be retained.

const authReq = req.clone({
    headers: req.headers.set('Content-Type', 'application/json')
    .set('header2', 'header 2 value')
    .set('header3', 'header 3 value')
});