<form [formGroup]="myForm" (ngSubmit)="onSubmit()">
  <div class="form-group">
    <label for="username">Username</label>
    <input
      type="text"
      name="username"
      class="form-control"
      formControlName="username"
    >
    <div *ngIf="myForm.controls.username.invalid" class="alert alert-danger">
      username is required
    </div>
  </div>

<form [formGroup]="myForm" (ngSubmit)="onSubmit()">
  <div class="form-group">
    <label for="username">Username</label>
    <input
      type="text"
      name="username"
      class="form-control"
      formControlName="username"
    >
    <div *ngIf="username.invalid" class="alert alert-danger">
      username is required
    </div>
  </div>

import {Component} from '@angular/core';
import {FormGroup, FormControl, Validators} from '@angular/forms';

@Component({
  selector: 'sign-up',
  templateUrl: 'app/sign-up.component.html'
})

export class SignUpComponent {

  myForm = new FormGroup({
    username: new FormControl('username', Validators.required),
    password: new FormControl('', Validators.required),
  });
}

export class SignUpComponent implements OnInit {

  myForm: FormGroup;
  usernameCtrl: FormControl;
  passwordCtrl: FormControl;

  ngOnInit() {
    this.usernameCtrl = new FormControl('username', Validators.required);
    this.passwordCtrl = new FormControl('', Validators.required);

    this.myForm = new FormGroup({
      username: this.usernameCtrl,
      password: this.passwordCtrl
    });
  }
}

export class SignUpComponent implements OnInit {

  myForm: FormGroup;
  usernameCtrl: FormControl;
  passwordCtrl: FormControl;

  constructor(private formBuilder: FormBuilder) { }

  ngOnInit() {
    this.usernameCtrl = this.formBuilder.control('username', Validators.required);
    this.passwordCtrl = this.formBuilder.control('', Validators.required);

    this.myForm = this.formBuilder.group({
      username: this.usernameCtrl,
      password: this.passwordCtrl
    });
  }
}

<div *ngIf="usernameCtrl.invalid" class="alert alert-danger">
  username is required
</div>

<input type="text"
       name="username"
       class="form-control"
       [formControl]="usernameCtrl">

usernameCtrl: FormControl;
passwordCtrl: FormControl;
...
this.usernameCtrl = this.formBuilder.control('username',Validators.required);
this.passwordCtrl = this.formBuilder.control('', Validators.required);

ngOnInit() {
this.myForm = this.formBuilder.group({
  username: ['usename', Validators.required]
  password: ['', Validators.required]
});

get username() { return this.myForm.get('username'); }
get password() { return this.myForm.get('password'); }

<div [formGroup]="myForm">
...
</div>

<input type="text"
       name="username"
       class="form-control"
       formControlName="username">
  <div *ngIf="username.invalid" class="alert alert-danger">
    username is required
  </div>

<input type="text"
       name="password"
       class="form-control"
       formControlName="password">
  <div *ngIf="password.invalid" class="alert alert-danger">
    password is required
  </div>