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

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;
  }

In the manual (https://angular.io/guide/http) I read: The HttpHeaders class is immutable, so every set() returns a new instance and applies the changes.

The following code works for me with angular-4:

 return this.http.get(url, {headers: new HttpHeaders().set('UserEmail', email ) });

First, you need to add HttpHeaders with HttpClient

import { HttpClient,HttpHeaders  } from '@angular/common/http';

your constructor should be like this.

constructor(private http: HttpClient) { }

then you can use like this

   let header = new HttpHeaders({ "Authorization": "Bearer "+token});

   const requestOptions = {  headers: header};                                                                                                                                                                            

    return this.http.get<any>(url, requestOptions)
        .toPromise()
        .then(data=> {
            //...
            return data;
    });

In my legacy app Array.from of prototype js was conflicting with angular's Array.from that was causing this problem. I resolved it by saving angular's Array.from version and reassigning it after prototype load.

I was struggling with this as well. I used an interceptor, it captures the response headers, then clone the headers(since headers are immutable objects) and then sends the modified headers. https://angular.io/guide/http#intercepting-requests-and-responses

Angular 8 HttpClient Service example with Error Handling and Custom Header

    import { Injectable } from '@angular/core';
    import { HttpClient, HttpHeaders, HttpErrorResponse } from '@angular/common/http';
    import { Student } from '../model/student';
    import { Observable, throwError } from 'rxjs';
    import { retry, catchError } from 'rxjs/operators';

    @Injectable({
      providedIn: 'root'
    })
    export class ApiService {

      // API path
      base_path = 'http://localhost:3000/students';

      constructor(private http: HttpClient) { }

      // Http Options
      httpOptions = {
        headers: new HttpHeaders({
          'Content-Type': 'application/json'
        })
      }

      // Handle API errors
      handleError(error: HttpErrorResponse) {
        if (error.error instanceof ErrorEvent) {
          // A client-side or network error occurred. Handle it accordingly.
          console.error('An error occurred:', error.error.message);
        } else {
          // The backend returned an unsuccessful response code.
          // The response body may contain clues as to what went wrong,
          console.error(
            `Backend returned code ${error.status}, ` +
            `body was: ${error.error}`);
        }
        // return an observable with a user-facing error message
        return throwError(
          'Something bad happened; please try again later.');
      };


      // Create a new item
      createItem(item): Observable<Student> {
        return this.http
          .post<Student>(this.base_path, JSON.stringify(item), this.httpOptions)
          .pipe(
            retry(2),
            catchError(this.handleError)
          )
      }

      ....
      ....

Check complete example tutorial here