Angular 7 - HttpClient "Property 'success' does not exist on type Object"

This is a compiler error, and it's one of the reasons Typescript is awesome!

Your registerUser method's return type is implicitly an Object (or {}) because that's what http.post is returning. http.post accepts a generic parameter to define what will be coming back in the response's body. Without that parameter, it will return type {} (because, without some sort of definition, JSON is just an unknown Object)... and a key of success does not exist on {}.

Assuming you have a fleshed out response object, just strongly type it:

interface UserPostResponse {
  success: boolean
}

...

  registerUser(user: User): Observable<UserPostResponse> {
    return this.http.post<UserPostResponse>(this.baseUri + '/register', user, { headers: this.headers });
  }

Conversely, if you wanted the HttpRequest itself and not the body, you just have to tell the HttpClient what part of the response to observe:

  registerUser(user: User): Observable<HttpResponse<object>> {
    return this.http.post(this.baseUri + '/register', user, { headers: this.headers }, observe: 'response');
  }

...and HttpResponse has a status, statusText, body, etc.

Property 'success' does not exist on type 'Object'. Create code as:

// Register User
this.apiService.registerUser(user).subscribe( (data) => {
    if(data['success']) {
        this.flashMessage.show('Registration successful', { cssClass: 'alert-success', timeout: 3200 });
    } else {
        this.flashMessage.show('Registration failed', { cssClass: 'alert-danger', timeout: 3200 });
    }
});

This change correct my error in Angular ts file.