Validators (required, minLength, pattern) in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a required validator to the form control.

Angular

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

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

Arequired

BminLength

Cpattern

DmaxLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using minLength or pattern instead of required.

The required validator ensures the form control is not empty.

2fill in blank

medium

Complete the code to add a minimum length validator of 5 characters.

Angular

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

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

A3

B8

C5

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a number smaller or larger than 5.

The minLength validator requires the input to be at least the given number of characters. Here, 5 means minimum 5 characters.

3fill in blank

hard

Fix the error in the pattern validator to only allow digits.

Angular

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

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

A^[a-zA-Z]+$

B^\\d+$

C^[a-z0-9]+$

D^\\w+$

Attempts:

3 left

💡 Hint

Common Mistakes

Using patterns that allow letters or other characters.

The pattern ^\\d+$ matches only digits from start to end.

4fill in blank

hard

Fill both blanks to create a form control with required and minimum length 8 validators.

Angular

this.form = new FormGroup({ password: new FormControl('', [Validators.[1], Validators.[2](8)]) });

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

Arequired

BminLength

Cpattern

DmaxLength

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order or using wrong validators.

The required validator ensures the field is filled, and minLength(8) ensures at least 8 characters.

5fill in blank

hard

Fill all three blanks to create a form control with required, minLength 6, and a pattern allowing only letters.

Angular

this.form = new FormGroup({ username: new FormControl('', [Validators.[1], Validators.[2](6), Validators.[3]('^[a-zA-Z]+$')]) });

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

Arequired

BminLength

Cpattern

DmaxLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong validator names or wrong pattern.

The validators ensure the username is filled, at least 6 characters, and contains only letters.

Practice

1. Which Angular validator ensures a form control is not left empty?

easy

A. Validators.pattern

B. Validators.minLength

C. Validators.required

D. Validators.maxLength

Solution

1. Step 1: Understand the purpose of Validators.required

   This validator checks if the input field has any value and marks it invalid if empty.

2. Step 2: Compare with other validators

   minLength checks length, pattern checks format, and maxLength limits length, but none enforce non-empty input.

3. Final Answer:

   Validators.required -> Option C

4. Quick Check:

   Non-empty input = Validators.required [OK]

Hint: Required means input cannot be empty [OK]

Common Mistakes:

• Confusing required with minLength
• Using pattern to check empty input
• Thinking maxLength enforces presence

2. Which is the correct syntax to apply a minimum length validator of 5 characters in Angular?

easy

A. Validators.minLength(5)

B. Validators.minlength = 5

C. Validators.minLength: 5

D. Validators.minLength[5]

Solution

1. Step 1: Recall Angular validator syntax

   Validators like minLength are functions called with the required parameter in parentheses.

2. Step 2: Identify correct function call

   Validators.minLength(5) correctly calls the function with 5 as argument; other options use invalid syntax.

3. Final Answer:

   Validators.minLength(5) -> Option A

4. Quick Check:

   Function call with parentheses = Validators.minLength(5) [OK]

Hint: Validators are functions called with parameters in parentheses [OK]

Common Mistakes:

• Using assignment (=) instead of function call
• Using colon (:) instead of parentheses
• Using brackets [] incorrectly

3. Given this form control:

new FormControl('', [Validators.required, Validators.minLength(3)])

What will be the validation status if the user enters 'Hi'?

medium

A. Valid

B. Invalid because input is empty

C. Invalid because input does not match pattern

D. Invalid because input length is less than 3

Solution

1. Step 1: Check input against Validators.required

   The input 'Hi' is not empty, so the required validator passes.

2. Step 2: Check input length against Validators.minLength(3)

   'Hi' has length 2, which is less than 3, so minLength validator fails.

3. Final Answer:

   Invalid because input length is less than 3 -> Option D

4. Quick Check:

   Length 2 < 3 = Invalid minLength [OK]

Hint: Check required first, then minLength [OK]

Common Mistakes:

• Assuming 'Hi' is valid because not empty
• Ignoring minLength validator
• Confusing pattern validator presence

4. Identify the error in this Angular form control validator setup:

new FormControl('', [Validators.required, Validators.pattern('[a-zA-Z]+'))

medium

A. Missing closing parenthesis for Validators.pattern

B. Pattern string should be a RegExp object, not a string

C. Validators.required cannot be combined with Validators.pattern

D. No error, this setup is correct

Solution

1. Step 1: Check syntax of Validators.pattern

   The pattern validator call has an opening parenthesis but is missing the closing parenthesis at the end.

2. Step 2: Verify other options

   Pattern can accept a string, and required can be combined with pattern, so options B and C are incorrect.

3. Final Answer:

   Missing closing parenthesis for Validators.pattern -> Option A

4. Quick Check:

   Unmatched parentheses cause syntax error [OK]

Hint: Count parentheses carefully in validator calls [OK]

Common Mistakes:

• Forgetting closing parenthesis
• Thinking pattern needs RegExp object only
• Believing required and pattern can't combine

5. You want a form control that requires input with at least 4 characters and only digits. Which validator setup is correct?

hard

A. [Validators.minLength(4), Validators.pattern('^[0-9]+$')]

B. [Validators.required, Validators.minLength(4), Validators.pattern('^[0-9]+$')]

C. [Validators.required, Validators.minLength(4), Validators.pattern('[a-zA-Z]+')]

D. [Validators.required, Validators.pattern('^[0-9]+$')]

Solution

1. Step 1: Check required presence

   The input must not be empty, so Validators.required is needed.

2. Step 2: Check length and pattern validators

   Validators.minLength(4) ensures at least 4 characters, and Validators.pattern('^[0-9]+$') ensures only digits.

3. Step 3: Evaluate other options

   [Validators.required, Validators.minLength(4), Validators.pattern('[a-zA-Z]+')] uses letters pattern, [Validators.minLength(4), Validators.pattern('^[0-9]+$')] misses required, [Validators.required, Validators.pattern('^[0-9]+$')] misses minLength(4).

4. Final Answer:

   [Validators.required, Validators.minLength(4), Validators.pattern('^[0-9]+$')] -> Option B

5. Quick Check:

   Required + minLength + digit pattern = [Validators.required, Validators.minLength(4), Validators.pattern('^[0-9]+$')] [OK]

Hint: Combine required, minLength, and digit pattern for digits only [OK]

Common Mistakes:

• Using letter pattern instead of digit pattern
• Omitting required validator
• Relying only on pattern for length