Form state tracking (dirty, touched, valid) in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the correct Angular form module.

Angular

import { [1] } from '@angular/forms';

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

AReactiveFormsModule

BHttpClientModule

CRouterModule

DCommonModule

Attempts:

3 left

💡 Hint

Common Mistakes

Importing HttpClientModule instead of ReactiveFormsModule

Forgetting to import any form module

The ReactiveFormsModule is needed to work with reactive forms and track form state like dirty, touched, and valid.

2fill in blank

medium

Complete the code to create a new form control with an initial empty value.

Angular

const nameControl = new FormControl([1]);

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

Anull

Bundefined

C''

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using null or undefined which can cause errors in template binding

Using 0 which is a number, not a string

Using an empty string '' initializes the form control with no text, which is common for text inputs.

3fill in blank

hard

Fix the error in checking if the form control has been touched.

Angular

if (this.form.get('email')?.[1]) { console.log('Email touched'); }

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

Atouched

Bpristine

Cdirty

Dvalid

Attempts:

3 left

💡 Hint

Common Mistakes

Using dirty instead of touched

Checking valid which is about validation, not interaction

The touched property tells if the user has focused and then left the input, which is what we want to check here.

4fill in blank

hard

Fill both blanks to create a form group with a required validator and check if it is valid.

Angular

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

const isValid = this.form.get('username')?.[2];

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

AValidators.required

BValidators.minLength(3)

Cvalid

Ddirty

Attempts:

3 left

💡 Hint

Common Mistakes

Using minLength validator instead of required

Checking dirty instead of valid

Use Validators.required to make the username mandatory, and check the valid property to see if the input passes validation.

5fill in blank

hard

Fill all three blanks to mark the form control as touched, check if it is dirty, and reset the form.

Angular

const control = this.form.get('password');
control?.[1]();
const isDirty = control?.[2];
this.form.[3]();

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

AmarkAsTouched

Bdirty

Creset

DmarkAsDirty

Attempts:

3 left

💡 Hint

Common Mistakes

Using markAsDirty instead of markAsTouched

Checking touched instead of dirty

Calling markAsTouched on the form instead of reset

Use markAsTouched() to mark the control as touched, check the dirty property to see if it was changed, and call reset() on the form to clear all controls.

Practice

1. In Angular forms, what does the dirty state indicate about a form control?

easy

A. The input has passed validation rules.

B. The user has changed the input value.

C. The user has focused and then left the input.

D. The form control is disabled.

Solution

1. Step 1: Understand the meaning of dirty in Angular forms

   The dirty state means the user has changed the value of the input from its original value.

2. Step 2: Compare with other states

   touched means the user focused and left the input, valid means the input passes validation, and disabled means the control is inactive.

3. Final Answer:

   The user has changed the input value. -> Option B

4. Quick Check:

   dirty = user changed input [OK]

Hint: Dirty means input value changed by user [OK]

Common Mistakes:

• Confusing dirty with touched state
• Thinking dirty means input is valid
• Assuming dirty means input is disabled

2. Which of the following is the correct way to check if a form control named email is touched in an Angular template?

easy

A. form.controls.email.touched

B. form.email.touched()

C. form.get('email').isTouched

D. form.controls['email'].isTouched()

Solution

1. Step 1: Recall Angular form control access syntax

   In Angular, form controls are accessed via form.controls.controlName or form.get('controlName'). The touched property is a boolean, not a method.

2. Step 2: Identify correct property usage

   form.controls.email.touched correctly accesses the touched property. Other options incorrectly use method calls or wrong property names.

3. Final Answer:

   form.controls.email.touched -> Option A

4. Quick Check:

   Access touched as property, not method [OK]

Hint: Use .controls.name.touched without parentheses [OK]

Common Mistakes:

• Using parentheses like a method on touched
• Using incorrect property names like isTouched
• Accessing controls incorrectly with dot vs bracket

3. Given this Angular reactive form control initialization:

this.form = new FormGroup({
  username: new FormControl('', Validators.required)
});

// User focuses on username input, types 'John', then leaves the input.

What will be the values of dirty, touched, and valid for username after these actions?

medium

A. dirty: true, touched: true, valid: false

B. dirty: false, touched: true, valid: true

C. dirty: true, touched: false, valid: false

D. dirty: true, touched: true, valid: true

Solution

1. Step 1: Analyze user actions on the form control

   User focused and left the input, so touched is true. User changed the value from empty string to 'John', so dirty is true.

2. Step 2: Check validation status

   The control has Validators.required. Since 'John' is not empty, the control is valid, so valid is true.

3. Final Answer:

   dirty: true, touched: true, valid: true -> Option D

4. Quick Check:

   Changed + left + valid input = dirty, touched, valid true [OK]

Hint: Changed input + left focus + valid value = all true [OK]

Common Mistakes:

• Assuming dirty is false if user leaves input
• Confusing touched with dirty
• Thinking valid is false if user typed anything

4. You have this Angular template snippet:

<input type="text" formControlName="email">
<div *ngIf="form.controls.email.dirty && form.controls.email.valid">Valid email changed!</div>

But the message never shows even after typing a valid email. What is the likely problem?

medium

A. The form control is not marked as touched, so the message won't show.

B. The valid property is misspelled and should be isValid.

C. The form control might have an email validator, so valid is false.

D. The dirty property does not update on user input.

Solution

1. Step 1: Understand the condition for showing the message

   The message shows only if the control is both dirty and valid.

2. Step 2: Check if validation is set up

   If the form control has an email validator but the input doesn't pass it (even if it looks valid), valid is false, so the message won't appear.

3. Final Answer:

   The form control might have an email validator, so valid is false. -> Option C

4. Quick Check:

   Missing validator means valid stays false [OK]

Hint: Check validators if valid never becomes true [OK]

Common Mistakes:

• Thinking touched affects this condition
• Using wrong property names like isValid
• Assuming dirty never updates on input

5. You want to disable the submit button in an Angular reactive form until the user has changed at least one input and all inputs are valid. Which condition correctly implements this logic in the template?

<button [disabled]="???">Submit</button>

hard

A. [disabled]="form.pristine || form.invalid"

B. [disabled]="!form.touched || !form.valid"

C. [disabled]="form.touched && form.valid"

D. [disabled]="!form.dirty && !form.valid"

Solution

1. Step 1: Understand form state properties

   pristine means no changes made, opposite of dirty. invalid means form is not valid.

2. Step 2: Translate requirement to condition

   Disable button if form is pristine (no changes) or invalid (not valid). So disable when form.pristine || form.invalid.

3. Final Answer:

   [disabled]="form.pristine || form.invalid" -> Option A

4. Quick Check:

   Disable if no changes or invalid [OK]

Hint: Disable if pristine or invalid to require change and validity [OK]

Common Mistakes:

• Using dirty instead of pristine incorrectly
• Confusing valid with invalid
• Using touched instead of dirty/pristine