Adding a HTTP header to the Angular HttpClient doesn't send the header, why?

The instances of the new HttpHeader class are immutable objects. Invoking class methods will return a new instance as result. So basically, you need to do the following:

let headers = new HttpHeaders();
headers = headers.set('Content-Type', 'application/json; charset=utf-8');

or

const headers = new HttpHeaders({'Content-Type':'application/json; charset=utf-8'});

Update: adding multiple headers

let headers = new HttpHeaders();
headers = headers.set('h1', 'v1').set('h2','v2');

or

const headers = new HttpHeaders({'h1':'v1','h2':'v2'});

Update: accept object map for HttpClient headers & params

Since 5.0.0-beta.6 is now possible to skip the creation of a HttpHeaders object an directly pass an object map as argument. So now its possible to do the following:

http.get('someurl',{
   headers: {'header1':'value1','header2':'value2'}
});

To add multiples params or headers you can do the following:

constructor(private _http: HttpClient) {}

//....

const url = `${environment.APP_API}/api/request`;

let headers = new HttpHeaders().set('header1', hvalue1); // create header object
headers = headers.append('header2', hvalue2); // add a new header, creating a new object
headers = headers.append('header3', hvalue3); // add another header

let params = new HttpParams().set('param1', value1); // create params object
params = params.append('param2', value2); // add a new param, creating a new object
params = params.append('param3', value3); // add another param 

return this._http.get<any[]>(url, { headers: headers, params: params })

set http headers like below in your http request

return this.http.get(url, { headers: new HttpHeaders({'Authorization': 'Bearer ' + token})
 });

I struggled with this for a long time. I am using Angular 6 and I found that

let headers = new HttpHeaders();
headers = headers.append('key', 'value');

did not work. But what did work was

let headers = new HttpHeaders().append('key', 'value');

did, which makes sense when you realize they are immutable. So having created a header you can't add to it. I haven't tried it, but I suspect

let headers = new HttpHeaders();
let headers1 = headers.append('key', 'value');

would work too.

I was with Angular 8 and the only thing which worked for me was this:

  getCustomHeaders(): HttpHeaders {
    const headers = new HttpHeaders()
      .set('Content-Type', 'application/json')
      .set('Api-Key', 'xxx');
    return headers;
  }