registration(event) {
    event.preventDefault();

    this.localStor();

    let newUserName = event.target.form.elements[0].value,
        newUserPass = event.target.form.elements[1].value;

    // берем значения с локал и сразу преобразуем
    let meaningsLocalS = JSON.parse(localStorage.getItem('user'));

    for (var prop in meaningsLocalS) {

        if (newUserName != prop.userName) {
            let newUser = {
                userName: newUserName,
                userPass: newUserPass
            }

            meaningsLocalS.push(newUser);

            let newJson = JSON.stringify(meaningsLocalS);
            localStorage.setItem('user', newJson);

            this.user.setUserLoggedIn();
            this.router.navigate(['/post']);

            break;
        } else {
            alert('Пользователь с таким логином существует.');
            break;
        }
    }



}

localStor() {

  const locSt = [
      {
          userName: "admin",
          userPass: "admin"
      },
      {
          userName: "marusya",
          userPass: "123"
      }
  ];
  let lc = JSON.stringify(locSt);
  localStorage.setItem('user', lc);
}

import { Injectable } from '@angular/core';

@Injectable()
export class UserService {

private isUserLoggedIn;
private userName;

  constructor() {
    this.isUserLoggedIn = false;
  }

    setUserLoggedIn() {
        this.isUserLoggedIn = true;
    }

    getUserLoggedIn() {
        return this.isUserLoggedIn;
    }

}