Form state tracking (dirty, touched, valid) in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'dirty' state mean in Angular forms?

The 'dirty' state means the user has changed the value of a form control since it was first displayed.

beginner

Explain the 'touched' state in Angular form controls.

A form control is 'touched' when the user has focused on it and then moved focus away (blurred).

beginner

What does it mean if a form control is 'valid'?

A form control is 'valid' if it passes all the validation rules set on it, like required fields or pattern matching.

intermediate

How can you check if a form is dirty in Angular?

You can check the form's 'dirty' property, for example: formGroup.dirty returns true if any control has been changed.

intermediate

Why is tracking 'touched' state useful in forms?

Tracking 'touched' helps show validation messages only after the user has interacted with a field, improving user experience.

Which Angular form state indicates the user has changed a field value?

A pristine

B touched

C valid

D dirty

The 'dirty' state means the user changed the value. 'Touched' means the field was focused and blurred.

What does the 'touched' state represent in Angular forms?

A Field has been focused and then blurred

B Field is valid

C Field value changed

D Field is empty

'Touched' means the user focused on the field and then moved away from it.

If a form control is valid, what does it mean?

A Control has been touched

B Control passes all validation rules

C User has typed something

D Control is empty

A valid control meets all validation requirements like required or pattern.

Which property would you check to see if a form has never been changed?

A dirty

B touched

C pristine

D valid

'Pristine' means the form or control has not been changed by the user.

Why might you wait to show validation errors until a control is 'touched'?

A To avoid showing errors before user interacts

B To mark the form as valid

C To reset the form

D To disable the submit button

Showing errors only after 'touched' improves user experience by not showing errors too early.

Describe the differences between 'dirty', 'touched', and 'valid' states in Angular forms.

Think about user actions and validation results.

How would you use form state tracking to improve user experience in a form?

Consider when to show errors and when to allow submission.

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