Showing validation errors in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to bind the form control to the input field.

Angular

<input type="text" [formControl]="[1]">

Drag options to blanks, or click blank then click option'

AcontrolName

BformGroup

CngModel

DnameControl

Attempts:

3 left

💡 Hint

Common Mistakes

Using formGroup instead of formControl

Using ngModel which is for template-driven forms

Using controlName which is for formGroup directives

The [formControl] directive binds the input to the specified FormControl instance.

2fill in blank

medium

Complete the code to check if the form control is invalid and touched.

Angular

<div *ngIf="[1].invalid && [1].touched">Please enter a valid value.</div>

Drag options to blanks, or click blank then click option'

AnameControl

BformGroup

CformControlName

DngModel

Attempts:

3 left

💡 Hint

Common Mistakes

Checking invalid on formGroup instead of formControl

Using formControlName string instead of variable

Using ngModel which is unrelated here

We check the invalid and touched properties on the FormControl instance to show errors only after user interaction.

3fill in blank

hard

Fix the error in the code to display the required error message.

Angular

<div *ngIf="[1].errors?.required">This field is required.</div>

Drag options to blanks, or click blank then click option'

AnameControl

BformGroup

CformControlName

DngModel

Attempts:

3 left

💡 Hint

Common Mistakes

Using formGroup instead of formControl

Using formControlName string instead of variable

Using ngModel which is unrelated here

The errors property is on the FormControl instance, so use the variable name.

4fill in blank

hard

Fill both blanks to create a form group with a required validator.

Angular

this.form = new FormGroup({ name: new FormControl('', [[1]]) });

<input type="text" [formControlName]="[2]">

Drag options to blanks, or click blank then click option'

AValidators.required

Bname

CValidators.minLength(3)

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using minLength instead of required validator

Mismatching formControlName string and FormGroup key

Using 'email' instead of 'name' for control name

The first blank needs the required validator function. The second blank is the control name string used in the template.

5fill in blank

hard

Fill all three blanks to show an error message only when the control is invalid, touched, and has a required error.

Angular

<div *ngIf="[1].invalid && [2] && [3].errors?.required">Name is required.</div>

Drag options to blanks, or click blank then click option'

AnameControl

BnameControl.touched

DnameControl.dirty

Attempts:

3 left

💡 Hint

Common Mistakes

Using dirty instead of touched

Using formGroup instead of formControl

Not checking errors?.required properly

The first and third blanks are the FormControl variable. The second blank checks if the control was touched.

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