Angular - ERROR ReferenceError: config is not defined

I'm following a tutorial of Angular 7 authentication and authorization from here.

I've added all the required files and there's no error during ng serve as well. The tutorial also requires

declare var config: any;

to be added to typings.d.ts file. By default there was no typings.d.ts file in my src directory so I manually created typings.d.ts and added the required declaration there. Config is being used in 2 services respectively

In Authentication.service.ts

login(username: string, password: string) {
        return this.http.post<any>(`${config.apiUrl}/users/authenticate`, { username, password })
            .pipe(map(user => {
                // login successful if there's a jwt token in the response
                if (user && user.token) {
                    // store user details and jwt token in local storage to keep user logged in between page refreshes
                    localStorage.setItem('currentUser', JSON.stringify(user));
                    this.currentUserSubject.next(user);
                }

                return user;
            }));

and in user.service.ts

 getAll() {
        return this.http.get<User[]>(`${config.apiUrl}/users`);
    }

    getById(id: number) {
        return this.http.get<User>(`${config.apiUrl}/users/${id}`);
    }

After adding the declaration in typings.d.ts file the error from compiler goes away but when I run the application and try to sign in I get the following error on console:

ERROR ReferenceError: config is not defined

Detailed log:

ERROR ReferenceError: config is not defined
    at AuthenticationService.push../src/app/auth/_services/authentication.service.ts.AuthenticationService.login (authentication.service.ts:23)
    at LoginComponent.push../src/app/login/login.component.ts.LoginComponent.onSubmit (login.component.ts:51)
    at Object.eval [as handleEvent] (LoginComponent.html:6)
    at handleEvent (core.js:10251)
    at callWithDebugContext (core.js:11344)
    at Object.debugHandleEvent [as handleEvent] (core.js:11047)
    at dispatchEvent (core.js:7710)
    at core.js:9190
    at SafeSubscriber.schedulerFn [as _next] (core.js:3563)
    at SafeSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.SafeSubscriber.__tryOrUnsub (Subscriber.js:195)

I've tried similar questions but they do not state any help. Any suggestions to fix or work around config in this regard will be highly appreciated. Thank you.

Entire code for authentication.service.ts

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { BehaviorSubject, Observable } from 'rxjs';
import { map } from 'rxjs/operators';

import { User } from '../_models';

@Injectable({ providedIn: 'root' })
export class AuthenticationService {
    private currentUserSubject: BehaviorSubject<User>;
    public currentUser: Observable<User>;

    constructor(private http: HttpClient) {
        this.currentUserSubject = new BehaviorSubject<User>(JSON.parse(localStorage.getItem('currentUser')));
        this.currentUser = this.currentUserSubject.asObservable();
    }

    public get currentUserValue(): User {
        return this.currentUserSubject.value;
    }

    login(username: string, password: string) {
        return this.http.post<any>(`${config.apiUrl}/users/authenticate`, { username, password })
            .pipe(map(user => {
                // login successful if there's a jwt token in the response
                if (user && user.token) {
                    // store user details and jwt token in local storage to keep user logged in between page refreshes
                    localStorage.setItem('currentUser', JSON.stringify(user));
                    this.currentUserSubject.next(user);
                }

                return user;
            }));
    }

    logout() {
        // remove user from local storage to log user out
        localStorage.removeItem('currentUser');
        this.currentUserSubject.next(null);
    }
}

Entire code for users.service.ts

import { AuthenticationService } from './authentication.service';
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { User } from '../_models';

@Injectable({ providedIn: 'root' })
export class UserService {
    constructor(private http: HttpClient) { }

    getAll() {
        return this.http.get<User[]>(`${config.apiUrl}/users`);
    }

    getById(id: number) {
        return this.http.get<User>(`${config.apiUrl}/users/${id}`);
    }
}