form-builder_validation.md

Validating FormBuilder Forms

Building from the previous login form, we can quickly and easily add validation.

Angular 2 provides many validators out of the box. They can be imported along with the rest of dependencies for procedural forms.

app/login-form.component.ts

import {
  // ...
  Validators
} from '@angular/forms';

@Component({ /* ... */ })
export class LoginForm {
  loginForm: FormGroup;
  username: FormControl;
  password: FormControl;

  constructor(builder: FormBuilder) {
    this.username = new FormControl('', [
      Validators.required,
      Validators.minLength(5)
    ]);
    this.password = new FormControl('', [Validators.required]);
    this.loginForm = builder.group({
      username: this.username,
      password: this.password
    });
  }

  login() {
    console.log(this.loginForm.value);
    // Attempt Logging in...
  }
}

app/login-form.component.html

<form [formGroup]="loginForm" (ngSubmit)="login()">
  Inside the form.
  <div>
    <label for="username">username</label>
    <input type="text" name="username" id="username" [formControl]="username">
    <div [hidden]="username.valid || username.untouched">
      <div>The following problems have been found with the username:</div>
      <div [hidden]="!username.hasError('minlength')">Username can not be shorter than 5 characters.</div>
      <div [hidden]="!username.hasError('required')">Username is required.</div>
    </div>
  </div>
  <div >
    <label for="password">password</label>
    <input type="password" name="password" id="password" [formControl]="password">
    <div [hidden]="password.valid || password.untouched">
      <div>The following problems have been found with the password:</div>
      <div [hidden]="!password.hasError('required')">The password is required.</div>
    </div>
  </div>
  <button type="submit" [disabled]="!loginForm.valid">Log In</button>
</form>

Note that we have added rather robust validation on both the fields and the form itself, using nothing more than built-in validators and some template logic.

View Example

We are using .valid and .untouched to determine if we need to show errors - while the field is required, there is no reason to tell the user that the value is wrong if the field hasn't been visited yet.

For built-in validation, we are calling .hasError() on the form element, and we are passing a string which represents the validator function we included. The error message only displays if this test returns true.