Angular Async Validators Tutorial 🎯

Welcome to the Angular Async Validators tutorial! In this lesson, we'll explore how to create and implement asynchronous validators in your Angular forms. We'll learn why we need async validators, how they work, and when to use them in practice.

What are Async Validators? 📝

Async validators are a powerful feature in Angular forms that allow us to validate user input by making a network request or performing a complex computation that takes longer than the typical synchronous validation.

Why use Async Validators? 💡

Validate user input against external APIs or services
Perform complex computations or data validation that may take time
Provide real-time feedback to users and enhance form usability

Setting Up Async Validators 🎯

To set up an async validator, we'll follow these steps:

Create a custom validator function
Register the validator with the FormBuilder
Use the validator in the reactive form

Step 1: Create a Custom Validator Function 📝

In Angular, we'll define an async validator function that returns an Observable<ValidationErrors | null>. This function will perform the validation and return an object containing error messages if validation fails or null if it passes.

typescript

import { AbstractControl, AsyncValidator, ValidationErrors } from '@angular/forms';

function checkUsernameAvailability(control: AbstractControl): Promise<ValidationErrors | null> {
  // Your async validation logic here
  // ...

  if (usernameIsTaken) {
    return of({ usernameTaken: true });
  }

  return of(null);
}

Step 2: Register the Validator with the FormBuilder 📝

Next, we'll register our custom async validator with the FormBuilder when creating a reactive form.

typescript

import { FormBuilder, Validators, AsyncValidatorFn, FormGroup } from '@angular/forms';
import { checkUsernameAvailability } from './username-validator';

this.userForm = this.fb.group({
  username: [
    '',
    [Validators.required, Validators.minLength(3)],
    checkUsernameAvailability
  ]
}, { validators: [validateAll] });

Step 3: Use the Validator in the Reactive Form 💡

In the template, we'll use the async validator in the form control, along with error handling.

html

<form [formGroup]="userForm">
  <mat-form-field>
    <input matInput [formControlName]="'username'" required>
    <mat-error *ngIf="userForm.get('username').hasError('usernameTaken')">
      Username is already taken. Please choose another one.
    </mat-error>
  </mat-form-field>

  <!-- Other form controls -->

  <button mat-raised-button color="primary" type="submit">Submit</button>
</form>

Practical Example 🎯

In this practical example, we'll create a simple user registration form that checks if the entered email is valid using an async validator.

Custom Validator Function 📝

typescript

import { AbstractControl, AsyncValidator, ValidationErrors } from '@angular/forms';
import { map, startWith } from 'rxjs/operators';
import { of } from 'rxjs';

function emailValidator(control: AbstractControl): Promise<ValidationErrors | null> {
  const emailRegex = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
  const email = control.value;

  if (emailRegex.test(email)) {
    return of(null);
  }

  return of({ emailInvalid: true });
}

Registering the Validator 📝

typescript

import { FormBuilder, Validators, AsyncValidatorFn, FormGroup } from '@angular/forms';
import { emailValidator } from './email-validator';

this.registrationForm = this.fb.group({
  email: ['', [Validators.required, emailValidator]]
}, { validators: [validateAll] });

Template 💡

html

<form [formGroup]="registrationForm">
  <mat-form-field>
    <input matInput [formControlName]="'email'" required>
    <mat-error *ngIf="registrationForm.get('email').hasError('emailInvalid')">
      Please enter a valid email address.
    </mat-error>
  </mat-form-field>

  <!-- Other form controls -->

  <button mat-raised-button color="primary" type="submit">Register</button>
</form>

Quiz 🎯

That's it for the Angular Async Validators tutorial! Now you can validate user input against external APIs or perform complex computations in your Angular forms. Happy coding! 🚀💻