login.component.ts

import { Component, OnInit } from '@angular/core';
import { AuthService } from '../services/auth.service';
import { Router } from '@angular/router'
import { UnauthError } from '../common/unauth-error'
import { AppError } from '../common/app-error'

@Component({
  selector: 'app-login',
  templateUrl: './login.component.html',
  styleUrls: ['./login.component.css']
})
export class LoginComponent implements OnInit {
  isInvalid: boolean = false

  constructor(
    private authService: AuthService,
    private router: Router) { }

  ngOnInit() {
  }

  submit(f) {
    const credentials = f.value

    this.authService.login(credentials)
      .subscribe(
        result => {
          this.router.navigate([''])
        },
        (error: AppError) => { // look here
          if (error instanceof UnauthError) {
            return this.isInvalid = true
          }
        }
      )
  }
}