Edit: Read the part at the end of the question!

I get this error:

My service code:

import { Http, Response, Headers } from "@angular/http";
import { Injectable } from "@angular/core";
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/catch';
import { Observable } from "rxjs";

import { Client } from "./client.model";

@Injectable()
export class ClientService {
    private clients: Client[] = [];

    constructor(private http: Http){}

    addClient(client: Client) {
        this.clients.push(client);
        const body = JSON.stringify(client);
        const headers = new Headers({'Content-Type': 'application/json'});
        return this.http.post('http://localhost:3000/client', body, {headers: headers})
            .map((response: Response) => response.json())
            .catch((error: Response) => Observable.throw(error.json()));
    }

    getClients() {
        return this.clients;
    }

    deleteClient(client: Client) {

    }
}

And in my component I have this submit function:

onSubmit(form: NgForm) {
    let email = form.value.email;
    let password = form.value.password;
    let firstName = form.value.firstName;
    let lastName = form.value.lastName;

    const client = new Client(email, password, firstName, lastName);
    this.clientService.addClient(client)
        .subscribe(
            data => console.log(data),
            error => console.error(error)
        );
    form.resetForm();
}

There are similar errors here and here. The answer is always to include the rxjs functions, but I do that, so I'm not quite sure, why I get this error.

Edit:

So the actual problem was not this function, but a missing "/" before my route in the main app.js file. After solving that, the issue was gone.