Showing validation errors in Angular - Mini Project: Build & Apply

Showing validation errors

📖 Scenario: You are building a simple Angular form for user registration. You want to show validation errors when the user leaves the input fields empty.

🎯 Goal: Create a standalone Angular component with a form that has two input fields: username and email. Show validation error messages below each input when the fields are empty and the user has touched them.

📋 What You'll Learn

Create a standalone Angular component named RegistrationFormComponent.

Add a form with two input fields: username and email.

Use Angular signals to track form control values and touched state.

Show validation error messages below each input when the field is empty and touched.

Use the new Angular 17+ control flow directives @if for conditional rendering.

💡 Why This Matters

🌍 Real World

Forms are everywhere on websites and apps. Showing validation errors helps users fix mistakes quickly and improves user experience.

💼 Career

Understanding Angular reactive forms and accessibility is essential for frontend developer roles working on user interfaces.

1

Set up the form controls

Create a standalone Angular component named RegistrationFormComponent. Inside the component, create two signals called username and email initialized with empty strings. Also create two signals called usernameTouched and emailTouched initialized with false.

Angular

import { Component, signal } from '@angular/core';

@Component({
  selector: 'app-registration-form',
  standalone: true,
  template: ``,
})
export class RegistrationFormComponent {
  // Your code here
}

Hint

Use signal from Angular to create reactive variables for the form fields and their touched states.

2

Add the form inputs and bind signals

In the component template, add two <input> elements for username and email. Bind their value to the respective signals and update the signals on input events. Also, on blur events, set the respective touched signals to true.

Angular

import { Component, signal } from '@angular/core';

@Component({
  selector: 'app-registration-form',
  standalone: true,
  template: `
    
  `,
})
export class RegistrationFormComponent {
  username = signal('');
  email = signal('');
  usernameTouched = signal(false);
  emailTouched = signal(false);
}

Hint

Use Angular property binding [value] and event binding (input) and (blur) to connect inputs to signals.

3

Add validation error messages

Below each input, use the Angular @if directive to show a <div> with the text Username is required if username is empty and usernameTouched is true. Similarly, show Email is required below the email input if email is empty and emailTouched is true.

Angular

import { Component, signal } from '@angular/core';

@Component({
  selector: 'app-registration-form',
  standalone: true,
  template: `
    Username:
    
    

    Email:
    
    
  `,
})
export class RegistrationFormComponent {
  username = signal('');
  email = signal('');
  usernameTouched = signal(false);
  emailTouched = signal(false);
}

Hint

Use Angular's *ngIf directive to conditionally show error messages based on the signals.

4

Make the component accessible and responsive

Add aria-label attributes to both inputs with values Username input and Email input. Wrap the form fields in a <form> element with aria-live="polite" to announce validation messages. Add simple CSS styles inside a <style> tag in the template to make inputs full width and error messages red and smaller.

Angular

import { Component, signal } from '@angular/core';

@Component({
  selector: 'app-registration-form',
  standalone: true,
  template: `
    
    
     tag -->
  `,
})
export class RegistrationFormComponent {
  username = signal('');
  email = signal('');
  usernameTouched = signal(false);
  emailTouched = signal(false);
}

Hint

Use aria-label on inputs for screen readers and wrap inputs in a <form> with aria-live="polite" to announce validation messages. Add CSS inside a <style> tag in the template.

Practice

1. In Angular, when should you show validation error messages for a form field?

easy

A. Only after the user has touched or changed the field and it is invalid

B. Immediately when the form loads, regardless of user interaction

C. Only after the form is submitted, ignoring field interaction

D. Always show errors even if the field is valid

Solution

1. Step 1: Understand user experience best practice

   Showing errors only after user interaction prevents confusion and clutter.

2. Step 2: Angular validation behavior

   Angular's touched or dirty flags help detect user interaction before showing errors.

3. Final Answer:

   Only after the user has touched or changed the field and it is invalid -> Option A

4. Quick Check:

   Show errors after user interaction = D [OK]

Hint: Show errors only when user touched and invalid [OK]

Common Mistakes:

• Showing errors immediately on page load
• Showing errors without checking if field is invalid
• Waiting only for form submit to show errors

2. Which Angular directive is used to check if a form control is invalid and touched in a template?

easy

A. *ngIf="myControl.invalid && myControl.touched"

B. *ngFor="let control of myControl.invalid"

C. *ngSwitchCase="myControl.valid"

D. *ngModel="myControl.errors"

Solution

1. Step 1: Identify Angular directives for conditionally showing elements

   *ngIf is used to conditionally display content based on an expression.

2. Step 2: Check correct condition for validation errors

   Use myControl.invalid && myControl.touched to show errors only when invalid and user touched the field.

3. Final Answer:

   *ngIf="myControl.invalid && myControl.touched" -> Option A

4. Quick Check:

   *ngIf with invalid and touched = A [OK]

Hint: Use *ngIf with invalid and touched to show errors [OK]

Common Mistakes:

• Using *ngFor instead of *ngIf for condition
• Checking valid instead of invalid
• Using *ngModel incorrectly for validation display

3. Given this Angular template snippet:

<input [formControl]="emailControl">
<div *ngIf="emailControl.invalid && emailControl.touched">
  <small *ngIf="emailControl.errors?.required">Email is required.</small>
  <small *ngIf="emailControl.errors?.email">Invalid email format.</small>
</div>

What will be shown if the user focuses and leaves the input empty?

medium

A. The message 'Invalid email format.' will be displayed

B. No message will be displayed because the field is empty

C. The message 'Email is required.' will be displayed

D. Both messages will be displayed

Solution

1. Step 1: Analyze validation conditions

   The input is empty and touched, so required error is true, but email error is false because empty triggers required first.

2. Step 2: Determine which message shows

   Only the required error message is shown because emailControl.errors?.required is true.

3. Final Answer:

   The message 'Email is required.' will be displayed -> Option C

4. Quick Check:

   Empty + touched triggers required error = B [OK]

Hint: Empty touched field triggers required error message [OK]

Common Mistakes:

• Assuming invalid email message shows on empty input
• Showing no message on touched empty field
• Showing both messages simultaneously

4. You wrote this Angular code to show a required error:

<div *ngIf="emailControl.errors.required">Email is required.</div>

But the error never shows even when the field is empty and touched. What is the likely problem?

medium

A. You need to add Validators.email to the control

B. You forgot to check if the control is touched before showing errors

C. You should use emailControl.invalid instead of emailControl.errors.required

D. You must use ngModel instead of formControl

Solution

1. Step 1: Understand when errors should show

   Errors should show only after user interaction, so checking touched is important.

2. Step 2: Identify missing condition

   The code only checks errors.required but not if the user touched the field, so error stays hidden initially.

3. Final Answer:

   You forgot to check if the control is touched before showing errors -> Option B

4. Quick Check:

   Check touched before errors to show messages = C [OK]

Hint: Always check touched before showing validation errors [OK]

Common Mistakes:

• Showing errors without user interaction check
• Confusing invalid with errors property
• Adding unrelated validators like email for required error

5. You want to show validation errors for a password field only if the user has interacted with it and it is invalid. You also want to show a custom message if the password is less than 8 characters. Which Angular template snippet correctly implements this?

hard

A. <input type="password" [formControl]="passwordControl">
<div *ngIf="passwordControl.invalid">
<small *ngIf="passwordControl.errors?.minlength">Password must be at least 8 characters.</small>
</div>

B. <input type="password" [formControl]="passwordControl">
<div *ngIf="passwordControl.errors?.minlength">
<small>Password must be at least 8 characters.</small>
</div>

C. <input type="password" [formControl]="passwordControl">
<div *ngIf="passwordControl.touched">
<small *ngIf="passwordControl.errors?.minlength">Password must be at least 8 characters.</small>
</div>

D. <input type="password" [formControl]="passwordControl">
<div *ngIf="passwordControl.invalid && passwordControl.touched">
<small *ngIf="passwordControl.errors?.minlength">Password must be at least 8 characters.</small>
</div>

Solution

1. Step 1: Ensure errors show only after user interaction

   Check both invalid and touched to avoid showing errors prematurely.

2. Step 2: Show specific minlength error message

   Use passwordControl.errors?.minlength inside the conditional block to display the custom message.

3. Final Answer:

   Correctly combines invalid, touched, and minlength error check -> Option D

4. Quick Check:

   Check invalid and touched with minlength error = A [OK]

Hint: Combine invalid and touched to show specific error messages [OK]

Common Mistakes:

• Showing errors without checking touched
• Showing errors on invalid without user interaction
• Missing specific error condition for minlength