Introduction

Angular is a development platform for building WEB, mobile and desktop applications using HTML, CSS and TypeScript (JavaScript). Currently, Angular is at version 13 and Google is the main maintainer of the project.

Form validation enables the quality, accuracy, and integrity of the user input data. It’s possible to validate a form in Angular in two ways, one with reactive forms (using functions in the component class) and the other with template-driven forms (using attributes in the HTML) and we will use the reactive forms way.

Prerequisites

Before you start, you need to install and configure the tools:

• git
• Node.js and npm
• Angular CLI
• IDE (e.g. Visual Studio Code or WebStorm)

Getting started

Create the Angular application

1. Let's create the application with the Angular base structure using the @angular/cli with the route file and the SCSS style format.

ng new angular-reactive-form-validation --routing true --style scss
CREATE angular-reactive-form-validation/README.md (1083 bytes)
CREATE angular-reactive-form-validation/.editorconfig (274 bytes)
CREATE angular-reactive-form-validation/.gitignore (548 bytes)
CREATE angular-reactive-form-validation/angular.json (3363 bytes)
CREATE angular-reactive-form-validation/package.json (1095 bytes)
CREATE angular-reactive-form-validation/tsconfig.json (863 bytes)
CREATE angular-reactive-form-validation/.browserslistrc (600 bytes)
CREATE angular-reactive-form-validation/karma.conf.js (1449 bytes)
CREATE angular-reactive-form-validation/tsconfig.app.json (287 bytes)
CREATE angular-reactive-form-validation/tsconfig.spec.json (333 bytes)
CREATE angular-reactive-form-validation/.vscode/extensions.json (130 bytes)
CREATE angular-reactive-form-validation/.vscode/launch.json (474 bytes)
CREATE angular-reactive-form-validation/.vscode/tasks.json (938 bytes)
CREATE angular-reactive-form-validation/src/favicon.ico (948 bytes)
CREATE angular-reactive-form-validation/src/index.html (315 bytes)
CREATE angular-reactive-form-validation/src/main.ts (372 bytes)
CREATE angular-reactive-form-validation/src/polyfills.ts (2338 bytes)
CREATE angular-reactive-form-validation/src/styles.scss (80 bytes)
CREATE angular-reactive-form-validation/src/test.ts (745 bytes)
CREATE angular-reactive-form-validation/src/assets/.gitkeep (0 bytes)
CREATE angular-reactive-form-validation/src/environments/environment.prod.ts (51 bytes)
CREATE angular-reactive-form-validation/src/environments/environment.ts (658 bytes)
CREATE angular-reactive-form-validation/src/app/app-routing.module.ts (245 bytes)
CREATE angular-reactive-form-validation/src/app/app.module.ts (393 bytes)
CREATE angular-reactive-form-validation/src/app/app.component.scss (0 bytes)
CREATE angular-reactive-form-validation/src/app/app.component.html (23364 bytes)
CREATE angular-reactive-form-validation/src/app/app.component.spec.ts (1151 bytes)
CREATE angular-reactive-form-validation/src/app/app.component.ts (237 bytes)
✔ Packages installed successfully.
    Successfully initialized git.

2. Install and configure the Bootstrap CSS framework. Do steps 2 and 3 of the post Adding the Bootstrap CSS framework to an Angular application.

3. Let's create a custom validator for the email field. Create the EmailValidatorDirective directive.

ng generate directive email-validator --skip-tests=true
CREATE src/app/email-validator.directive.ts (157 bytes)
UPDATE src/app/app.module.ts (592 bytes)

4. Change the src/app/email-validator.directive.ts file. Create the emailValidator function and the EmailValidatorDirective class as below.

import { Directive } from '@angular/core';
import { NG_VALIDATORS, AbstractControl, Validator, ValidationErrors, ValidatorFn } from '@angular/forms';

export function emailValidator(): ValidatorFn {

  const EMAIL_REGEXP = /^(([^<>()\[\]\.,;:\s@\"]+(\.[^<>()\[\]\.,;:\s@\"]+)*)|(\".+\"))@(([^<>()[\]\.,;:\s@\"]+\.)+[^<>()[\]\.,;:\s@\"]{2,})$/i;

  return (control: AbstractControl): ValidationErrors | null => {
    const isValid = EMAIL_REGEXP.test(control.value);

    if (isValid) {
      return null;
    } else {
      return {
        emailValidator: {
          valid: false,
        },
      };
    }
  };

}

@Directive({
  selector: '[appEmailValidator]',
  providers: [{
    provide: NG_VALIDATORS,
    useExisting: EmailValidatorDirective,
    multi: true,
  }],
})
export class EmailValidatorDirective implements Validator {

  constructor() {
  }

  public validate(control: AbstractControl): ValidationErrors | null {
    return emailValidator()(control);
  }

}

5. Change the src/app/app.component.ts file. Import the NgForm service, create the IUser interface and create the validate function as below.

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

import { emailValidator } from './email-validator.directive';

interface IUser {
  name: string;
  nickname: string;
  email: string;
  password: string;
  showPassword: boolean;
}

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit {

  reactiveForm!: FormGroup;
  user: IUser;

  constructor() {
    this.user = {} as IUser;
  }

  ngOnInit(): void {
    this.reactiveForm = new FormGroup({
      name: new FormControl(this.user.name, [
        Validators.required,
        Validators.minLength(1),
        Validators.maxLength(250),
      ]),
      nickname: new FormControl(this.user.nickname, [
        Validators.maxLength(10),
      ]),
      email: new FormControl(this.user.email, [
        Validators.required,
        Validators.minLength(1),
        Validators.maxLength(250),
        emailValidator(),
      ]),
      password: new FormControl(this.user.password, [
        Validators.required,
        Validators.minLength(15),
      ]),
    });
  }

  get name() {
    return this.reactiveForm.get('name')!;
  }

  get nickname() {
    return this.reactiveForm.get('nickname')!;
  }

  get email() {
    return this.reactiveForm.get('email')!;
  }

  get password() {
    return this.reactiveForm.get('password')!;
  }

  public validate(): void {
    if (this.reactiveForm.invalid) {
      for (const control of Object.keys(this.reactiveForm.controls)) {
        this.reactiveForm.controls[control].markAsTouched();
      }
      return;
    }

    this.user = this.reactiveForm.value;

    console.info('Name:', this.user.name);
    console.info('Nickname:', this.user.nickname);
    console.info('Email:', this.user.email);
    console.info('Password:', this.user.password);
  }

}

6. Change the src/app/app.component.html file. Add the form as below.

<div class="container-fluid py-3">
  <h1>Angular Reactive Form Validation</h1>

  <div class="row justify-content-center my-5">
    <div class="col-4">
      <form [formGroup]="reactiveForm" #form="ngForm">
        <div class="row">
          <div class="col mb-2">
            <label for="name" class="form-label">Name:</label>
            <input type="text" id="name" name="name" formControlName="name" placeholder="Your name" required minlength="1" maxlength="250" class="form-control form-control-sm" [class.is-invalid]="name.invalid && (name.dirty || name.touched)">
            <div *ngIf="name.invalid && (name.dirty || name.touched)" class="invalid-feedback">
              <div *ngIf="name.errors?.['required']">
                This field is required.
              </div>
              <div *ngIf="name.errors?.['minlength']">
                This field must have at least 1 character.
              </div>
              <div *ngIf="name.errors?.['maxlength']">
                This field must have at most 250 characters.
              </div>
            </div>
          </div>
        </div>
        <div class="row">
          <div class="col mb-2">
            <label for="nickname" class="form-label">Nickname:</label>
            <input type="text" id="nickname" name="nickname" formControlName="nickname" placeholder="Your nickname" maxlength="10" class="form-control form-control-sm" [class.is-invalid]="nickname.invalid && (nickname.dirty || nickname.touched)">
            <div *ngIf="nickname.invalid && (nickname.dirty || nickname.touched)" class="invalid-feedback">
              <div *ngIf="nickname.errors?.['maxlength']">
                This field must have at most 10 characters.
              </div>
            </div>
          </div>
        </div>
        <div class="row">
          <div class="col mb-2">
            <label for="email" class="form-label">Email:</label>
            <input type="email" id="email" name="email" formControlName="email" placeholder="your-name@provider.com" required minlength="1" maxlength="250" class="form-control form-control-sm" [class.is-invalid]="email.invalid && (email.dirty || email.touched)">
            <div *ngIf="email.invalid && (email.dirty || email.touched)" class="invalid-feedback">
              <div *ngIf="email.errors?.['required']">
                This field is required.
              </div>
              <div *ngIf="email.errors?.['minlength']">
                This field must have at least 1 character.
              </div>
              <div *ngIf="email.errors?.['maxlength']">
                This field must have at most 250 characters.
              </div>
              <div *ngIf="!email.errors?.['required'] && !email.errors?.['minlength'] && !email.errors?.['maxlength'] && email.errors?.['emailValidator']">
                Invalid email format.
              </div>
            </div>
          </div>
        </div>
        <div class="row">
          <div class="col mb-2">
            <label for="password" class="form-label">Password:</label>
            <div class="input-group input-group-sm has-validation">
              <input [type]="user.showPassword ? 'text' : 'password'" id="password" name="password" formControlName="password" required minlength="15" class="form-control form-control-sm" [class.is-invalid]="password.invalid && (password.dirty || password.touched)">
              <button type="button" class="btn btn-outline-secondary" (click)="user.showPassword = !user.showPassword">
                <i class="bi" [ngClass]="{'bi-eye-fill': !user.showPassword, 'bi-eye-slash-fill': user.showPassword}"></i>
              </button>
              <div *ngIf="password.invalid && (password.dirty || password.touched)" class="invalid-feedback">
                <div *ngIf="password.errors?.['required']">
                  This field is required.
                </div>
                <div *ngIf="password.errors?.['minlength']">
                  This field must have at least 15 characters.
                </div>
              </div>
            </div>
          </div>
        </div>
        <div class="row">
          <div class="col mb-2 d-grid">
            <button type="button" class="btn btn-sm btn-primary" (click)="validate()">Validate</button>
          </div>
        </div>
      </form>
    </div>
  </div>
</div>

7. Change the src/app/app.module.ts file. Import the ReactiveFormsModule module and the EmailValidatorDirective directive as below.

import { ReactiveFormsModule } from '@angular/forms';

import { EmailValidatorDirective } from './email-validator.directive';

declarations: [
  AppComponent,
  EmailValidatorDirective,
],
imports: [
  BrowserModule,
  ReactiveFormsModule,
  AppRoutingModule,
],

8. Run the application with the command below.

npm start

> angular-reactive-form-validation@1.0.0 start
> ng serve

✔ Browser application bundle generation complete.

Initial Chunk Files   | Names         |  Raw Size
vendor.js             | vendor        |   2.25 MB | 
styles.css, styles.js | styles        | 454.68 kB | 
polyfills.js          | polyfills     | 294.84 kB | 
scripts.js            | scripts       |  76.33 kB | 
main.js               | main          |  27.76 kB | 
runtime.js            | runtime       |   6.56 kB | 

                      | Initial Total |   3.09 MB

Build at: 2022-03-21T00:40:49.519Z - Hash: af669f909d9510f8 - Time: 3254ms

** Angular Live Development Server is listening on localhost:4200, open your browser on http://localhost:4200/ **


✔ Compiled successfully.

9. Ready! Access the URL http://localhost:4200/ and check if the application is working. See the application working on GitHub Pages and Stackblitz.

The application repository is available at https://github.com/rodrigokamada/angular-reactive-form-validation.

This tutorial was posted on my blog in portuguese.