How to add multiple headers in Angular 5 HttpInterceptor

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')
});