Error:(27, 16) TS2322: Type 'Observable<void>' is not assignable to type 'Observable<User>'
Type 'void' is not assignable to type 'User'.

export class User {
    constructor(
        public id:number,
        public name:string,
        public username:string,
        public password:string
    ) { }
}

import {Injectable} from 'angular2/core';
import {Headers, RequestOptions} from 'angular2/http';
import {Observable} from "rxjs/Observable";
import {User} from "./../user";
import {Http, Response} from 'angular2/http';

@Injectable()
export class LoginService {

    constructor (private http: Http) {}

    private _user_session_url = 'auth/sign_in';

    token:string;
    expiry:string;
    client:string;
    uid:string;

    login (username: string, password: string) : Observable<User> {
        let body = JSON.stringify({ username: username, password: password });
        let headers = new Headers({
            'Content-Type': 'application/json',
            'accept': 'json'
        });
        let options = new RequestOptions({ headers: headers});

        return this.http.post(this._user_session_url, body, options).map(this.extractData).catch(this.handleError)
    }

    private extractData(res: Response) {
        if (res.status < 200 || res.status >= 300) {
            throw new Error('Bad response status: ' + res.status);
        }

        let data = res.json();
        console.log(data);
        this.token = data.token;
        this.expiry = data.expiry;
        this.client = data.client;
        localStorage.setItem('access-token', this.token);
        localStorage.setItem('client', this.client);
        localStorage.setItem('expiry', this.expiry);
        localStorage.setItem('uid', this.uid);
    }

    private handleError (error: any) {
        let errMsg = error.json().error || 'Server error';
        console.error(errMsg); // log to console instead
        return Observable.throw(errMsg);
    }
}