Validators (required, minLength, pattern) in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a required validator is applied to an empty input?

Consider an Angular form control with the Validators.required applied. What will be the validation status if the input is empty?

Angular

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

console.log(this.form.controls['name'].valid);

A The control throws an error due to missing input.

B The control is valid because empty string is allowed.

C The control is invalid because the input is empty.

D The control is pending validation until user types.

💡 Hint

Think about what 'required' means for a form control value.

The Validators.required ensures the input is not empty. If the input is empty, the control is invalid.

📝 Syntax

intermediate

Which option correctly applies minLength validator to a form control?

You want to require that a username input has at least 5 characters. Which code correctly applies the Validators.minLength?

A new FormControl('', Validators.minLength(5))

B new FormControl('', Validators.minLength = 5)

C new FormControl('', Validators.minLength[5])

D new FormControl('', Validators.minLength{5})

💡 Hint

Remember how to call a validator function with a parameter.

The Validators.minLength is a function that takes a number and returns a validator function. It must be called as Validators.minLength(5).

🔧 Debug

advanced

Why does this pattern validator not work as expected?

Given this form control with a pattern validator to accept only digits:

new FormControl('', Validators.pattern('[0-9]+'))

Why might it fail to validate a string of digits?

A The pattern string should be wrapped with start and end anchors: '^' and '$'.

B The pattern should use double backslashes for digits: '\\d+'.

C The pattern validator only works with letters, not digits.

D The pattern must be passed as a RegExp object, not a string.

💡 Hint

Think about what the pattern matches by default without anchors.

Without anchors, the pattern matches anywhere in the string. To ensure the entire input matches digits only, use '^' and '$' anchors.

❓ state_output

advanced

What is the validation error object when minLength fails?

If a form control has Validators.minLength(4) and the input is 'abc', what is the value of control.errors?

Angular

const control = new FormControl('abc', Validators.minLength(4));
console.log(control.errors);

A null

B { 'minlength': { 'requiredLength': 4, 'actualLength': 3 } }

C { 'minLength': { 'requiredLength': 4, 'actualLength': 3 } }

D { 'lengthError': true }

💡 Hint

Check the exact error key and properties returned by Angular's minLength validator.

The error object key is 'minlength' (all lowercase), and it contains requiredLength and actualLength properties.

🧠 Conceptual

expert

How do multiple validators combine in Angular forms?

If a form control has Validators.required and Validators.pattern('^[a-z]+$') applied together, what is the validation result when the input is an empty string?

Angular

const control = new FormControl('', [Validators.required, Validators.pattern('^[a-z]+$')]);
console.log(control.errors);

A { 'pattern': { 'requiredPattern': '^[a-z]+$', 'actualValue': '' } }

B null

C { 'required': true, 'pattern': { 'requiredPattern': '^[a-z]+$', 'actualValue': '' } }

D { 'required': true }

💡 Hint

Think about which validator fails first and how Angular reports errors.

When the input is empty, Validators.required fails, so the error object contains only { required: true }. The pattern validator is not checked if required fails.

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