Form validation with template attributes in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when the user submits this Angular template-driven form?

Consider this Angular template snippet for a simple form:

<form #myForm="ngForm" (ngSubmit)="onSubmit(myForm)">
  <input name="email" ngModel required email />
  <button type="submit">Submit</button>
</form>

What will be the state of myForm.valid if the input is empty and the user clicks Submit?

Angular

<form #myForm="ngForm" (ngSubmit)="onSubmit(myForm)">
  <input name="email" ngModel required email />
  <button type="submit">Submit</button>
</form>

A myForm.valid will be false because the required field is empty.

B myForm.valid will be true because the form submits anyway.

C myForm.valid will be undefined because no validation runs on submit.

D myForm.valid will be true only if the input has any text, even if not an email.

💡 Hint

Think about what the required attribute does in Angular forms.

The required attribute marks the input as mandatory. If empty, Angular sets myForm.valid to false, preventing submission.

📝 Syntax

intermediate

Which template attribute correctly validates a minimum length of 5 characters?

In Angular template-driven forms, which attribute correctly enforces a minimum length of 5 characters on an input?

Angular

<input name="username" ngModel ... />

A min="5"

B minLength="5"

C minlength="5"

D min_length="5"

💡 Hint

Angular uses standard HTML5 validation attributes for length.

The correct attribute is minlength all lowercase. Angular recognizes it for validation.

🔧 Debug

advanced

Why does this Angular form input not validate the email correctly?

Look at this input in an Angular template-driven form:

<input name="userEmail" ngModel required type="email" />

Despite entering invalid emails, the form shows userEmail.valid as true. What is the likely cause?

Angular

<input name="userEmail" ngModel required type="email" />

A The input is missing the <code>email</code> attribute for Angular validation.

B The <code>type="email"</code> attribute disables Angular validation.

C The <code>required</code> attribute conflicts with <code>type="email"</code>.

D Angular does not support email validation in template-driven forms.

💡 Hint

Angular template-driven forms need a specific attribute for email validation.

Angular requires the email attribute on inputs to apply its email validator. Just type="email" is not enough.

❓ state_output

advanced

What is the value of myForm.controls['password'].errors after input?

Given this Angular template-driven form input:

<input name="password" ngModel required minlength="8" />

If the user types "abc" in the password field, what will myForm.controls['password'].errors contain?

Angular

<input name="password" ngModel required minlength="8" />

A null

B {"minlength": {"requiredLength": 8, "actualLength": 3}}

C {"required": true}

D {"maxlength": {"requiredLength": 8, "actualLength": 3}}

💡 Hint

Think about what errors Angular reports when minlength is not met.

Angular sets the minlength error with details about required and actual lengths when input is too short.

🧠 Conceptual

expert

Why use template attributes for validation instead of reactive forms validators?

In Angular, template-driven forms use validation attributes like required and minlength directly in the template. Reactive forms use validator functions in the component code.

What is a key advantage of using template attributes for validation?

A They support asynchronous validation out of the box.

B They provide better performance than reactive form validators.

C They enable validation logic to run on the server automatically.

D They allow quick, declarative validation without writing extra TypeScript code.

💡 Hint

Think about simplicity and where validation rules live.

Template attributes let you add validation rules directly in HTML, making it easier and faster to set up simple validations without extra code.

Practice

1. What is the main purpose of using template attributes like required or minlength in Angular forms?

easy

A. To write complex validation logic in the component class

B. To add simple validation rules directly in the HTML template

C. To style the form inputs with CSS

D. To handle form submission events

Solution

1. Step 1: Understand template attributes role

   Template attributes like required and minlength are used in the HTML to set validation rules simply and declaratively.

2. Step 2: Differentiate from other form tasks

   These attributes do not handle styling, complex logic, or submission events but focus on validation rules in the template.

3. Final Answer:

   To add simple validation rules directly in the HTML template -> Option B

4. Quick Check:

   Template attributes = simple validation rules [OK]

Hint: Template attributes set validation rules in HTML, not logic or style [OK]

Common Mistakes:

• Confusing validation attributes with styling
• Thinking validation logic must be in the component class
• Assuming template attributes handle form submission

2. Which of the following is the correct way to mark an input as required using template attributes in Angular?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall standard HTML validation syntax

   The required attribute is a standard HTML attribute used as required without brackets or extra directives.

2. Step 2: Identify Angular template attribute usage

   Angular supports standard HTML validation attributes directly in the template, so required alone is correct.

3. Final Answer:

   <input type="text" required> -> Option C

4. Quick Check:

   Use plain required attribute for required fields [OK]

Hint: Use plain required attribute without brackets for template validation [OK]

Common Mistakes:

• Using Angular binding syntax for static required attribute
• Confusing Angular directives with HTML attributes
• Adding non-existent attributes like validate-required

3. Given this Angular template snippet:

<input type="text" name="username" ngModel required minlength="4" #usernameRef="ngModel">
<div *ngIf="usernameRef.invalid && usernameRef.touched">
  <small *ngIf="usernameRef.errors?.required">Username is required.</small>
  <small *ngIf="usernameRef.errors?.minlength">Minimum 4 characters.</small>
</div>

What will happen if the user types "abc" and then clicks outside the input?

medium

A. The message "Minimum 4 characters." will be shown

B. No message will be shown because the input is valid

C. The message "Username is required." will be shown

D. Both messages will be shown

Solution

1. Step 1: Check input value against validation rules

   The input value "abc" has 3 characters, which is less than the minlength of 4, so minlength validation fails.

2. Step 2: Determine which error messages show on invalid and touched

   The input is not empty, so required is satisfied. Since usernameRef.invalid and usernameRef.touched are true, only the minlength error message appears.

3. Final Answer:

   The message "Minimum 4 characters." will be shown -> Option A

4. Quick Check:

   Input too short = minlength error shown [OK]

Hint: Check which validation fails for input value and touched state [OK]

Common Mistakes:

• Showing required error when input is not empty
• Showing no message when input is invalid and touched
• Showing both errors when only one applies

4. Identify the error in this Angular template snippet for form validation:

<input type="email" name="email" ngModel required #emailRef>
<div *ngIf="emailRef.invalid && emailRef.touched">
  <small *ngIf="emailRef.errors.required">Email is required.</small>
  <small *ngIf="emailRef.errors.email">Invalid email format.</small>
</div>

medium

A. The input type should be "text" instead of "email"

B. The required attribute should be [required]="true"

C. The *ngIf conditions should use OR (||) instead of AND (&&)

D. Missing template reference variable export like #emailRef="ngModel"

Solution

1. Step 1: Check template reference variable usage

   The template reference variable #emailRef is used to access validation state, but it lacks the export ="ngModel" which is needed to access invalid and errors.

2. Step 2: Validate other parts

   The required attribute is correct as is, AND logic for showing errors is appropriate, and input type "email" is valid for email validation.

3. Final Answer:

   Missing template reference variable export like #emailRef="ngModel" -> Option D

4. Quick Check:

   Template ref must export ngModel for validation access [OK]

Hint: Always export ngModel in template ref to access validation state [OK]

Common Mistakes:

• Omitting ="ngModel" in template reference variable
• Changing required attribute to binding unnecessarily
• Misusing logical operators in *ngIf
• Changing input type incorrectly

5. You want to disable the submit button until the form is valid using template-driven validation. Which of these is the best way to do it?

<form #myForm="ngForm">
  <input name="email" ngModel required email>
  <button type="submit" [disabled]="???">Submit</button>
</form>

What should replace ??? to disable the button until the form is valid?

hard

A. myForm.invalid

B. myForm.valid

C. myForm.touched

D. !myForm.pristine

Solution

1. Step 1: Understand form validity properties

   The form object has a property invalid which is true when the form is not valid, and valid which is true when valid.

2. Step 2: Choose the best expression to disable button

   Disabling the button when myForm.invalid is true means the button is disabled until the form becomes valid. Using myForm.valid would disable when valid (incorrect). Other options relate to touch or pristine state, not validity.

3. Final Answer:

   myForm.invalid -> Option A

4. Quick Check:

   Disable button when form.invalid is true [OK]

Hint: Use form.invalid to disable submit until form is valid [OK]

Common Mistakes:

• Using myForm.valid instead of myForm.invalid
• Disabling based on touched or pristine instead of validity
• Confusing valid and invalid properties