Showing validation errors in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of showing validation errors in Angular forms?

Showing validation errors helps users understand what input is required or incorrect, guiding them to fix mistakes before submitting the form.

beginner

How do you check if a form control has an error in Angular?

You check if the control is invalid and if it has been touched or dirty using properties like control.invalid and control.touched or control.dirty.

beginner

What Angular directive helps display validation errors conditionally in templates?

The *ngIf directive is used to show validation error messages only when the control has errors and the user has interacted with it.

intermediate

Why is it important to check if a control is touched or dirty before showing errors?

To avoid showing errors before the user interacts with the form, which can be confusing or overwhelming. It improves user experience by showing errors only after input.

intermediate

How can you show different error messages for different validation errors on the same form control?

By checking specific error keys on the control's errors object, like required, minlength, or pattern, and displaying messages accordingly.

Which Angular property indicates if a form control has failed validation?

A valid

B invalid

C pristine

D untouched

The invalid property is true when the control fails validation.

What does the *ngIf directive do when showing validation errors?

A It submits the form

B It styles the form control

C It conditionally displays error messages based on control state

D It disables the form control

*ngIf shows or hides elements based on a condition, useful for showing errors only when needed.

Why should you check if a control is touched before showing errors?

A To reset the form

B To clear the input

C To disable the submit button

D To show errors only after user interaction

Checking touched ensures errors appear only after the user has tried to change the input.

Which of these is NOT a common validation error key in Angular?

A maxvalue

B minlength

C required

D pattern

maxvalue is not a standard Angular validation error key; max is used instead.

How do you access the errors of a form control in Angular?

A control.errors

B control.value

C control.status

D control.name

The errors property contains the validation errors for the control.

Explain how to show validation error messages for a form control in Angular templates.

Think about when and how to show messages only after user interaction.

Describe why it is important to check user interaction states like touched or dirty before showing validation errors.

Consider how users feel if errors appear immediately on page load.

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