usernameEmailValidator.ts

import {Http, HTTP_PROVIDERS} from 'angular2/http';
import {Injector} from 'angular2/core'
import {Control} from 'angular2/common';
import 'rxjs/Rx';

export class UsernameEmailValidator {
  
  constructor() {}
  
  static checkUser(control: Control) {
    
    let injector = Injector.resolveAndCreate([HTTP_PROVIDERS]);
    var http = injector.get(Http);
    
    control.valueChanges
      .debounceTime(500)
      .flatMap(value => http.post('http://localhost:3000/api/users/check', JSON.stringify({ username: value })))
      .subscribe(
        data => {
          return null;
        },
        error => {
          return {'usernameTaken': true};
        }
      );

  }
}