Form validation patterns in React Native - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a state variable for the email input using React Native hooks.

React Native

const [email, [1]] = useState('');

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

AupdateEmail

BemailSet

CsetEmail

DchangeEmail

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that does not start with 'set' can confuse the code and cause errors.

In React Native, useState returns a pair: the current state and a function to update it. The convention is to name the updater function starting with 'set'.

2fill in blank

medium

Complete the code to check if the email input contains an '@' symbol for basic validation.

React Native

const isValidEmail = email.includes([1]);

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

A'#'

B'@'

C'$'

D'!'

Attempts:

3 left

💡 Hint

Common Mistakes

Using characters other than '@' will not correctly validate the email.

Checking if the email contains '@' is a simple way to validate email format.

3fill in blank

hard

Fix the error in the code to update the password state when the input changes.

React Native

<TextInput value={password} onChangeText=[1] />

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

Apassword

BupdatePassword

Cpassword()

DsetPassword

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the state variable instead of the updater function causes the input not to update.

The onChangeText prop expects a function that updates the state. 'setPassword' is the correct updater function.

4fill in blank

hard

Fill both blanks to create a simple validation that checks if the password length is at least 8 characters.

React Native

const isPasswordValid = password.length [1] [2];

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

A>=

B<

C8

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>=' reverses the logic and causes wrong validation.

To check if password length is at least 8, use 'password.length >= 8'.

5fill in blank

hard

Fill all three blanks to create an object that stores validation errors for email and password.

React Native

const errors = { email: [1], password: [2], isValid: [3] };

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

A''

Bnull

Cfalse

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using null instead of empty strings can cause UI issues displaying errors.

Empty strings '' mean no error messages for email and password. 'false' means the form is not valid yet.

Practice

1. What is the main purpose of form validation in a React Native app?

easy

A. To check user input and show errors if needed

B. To change the app's theme colors

C. To speed up the app loading time

D. To add animations to buttons

Solution

1. Step 1: Understand form validation role

   Form validation is used to check if user input is correct or complete.

2. Step 2: Identify the correct purpose

   Only To check user input and show errors if needed describes checking input and showing errors, which is the main goal.

3. Final Answer:

   To check user input and show errors if needed -> Option A

4. Quick Check:

   Form validation = check input and show errors [OK]

Hint: Form validation means checking input for mistakes [OK]

Common Mistakes:

• Confusing validation with UI styling
• Thinking validation speeds app loading
• Mixing validation with animations

2. Which of the following is the correct way to define a simple validation function in React Native?

easy

A. validate = (text) => { text.length > 0 ? 'Error' : ''; }

B. function validate(text) { return text.length < 0 ? 'Error' : ''; }

C. const validate = (text) => text.length > 0 ? '' : 'Required';

D. const validate = text => { if text.length = 0 return 'Error'; }

Solution

1. Step 1: Check syntax correctness

   const validate = (text) => text.length > 0 ? '' : 'Required'; uses correct arrow function syntax and returns empty string if valid.

2. Step 2: Identify logical correctness

   const validate = (text) => text.length > 0 ? '' : 'Required'; returns 'Required' if text is empty, which is expected behavior.

3. Final Answer:

   const validate = (text) => text.length > 0 ? '' : 'Required'; -> Option C

4. Quick Check:

   Correct arrow function with conditional return = const validate = (text) => text.length > 0 ? '' : 'Required'; [OK]

Hint: Arrow functions with conditional return are common for validation [OK]

Common Mistakes:

• Using assignment '=' instead of comparison '==' or '==='
• Missing return statements in function
• Incorrect arrow function syntax

3. Given this code snippet, what will be the value of error after calling validate('')?

const validate = (text) => {
  if (text.trim() === '') {
    return 'Field cannot be empty';
  }
  return '';
};
const error = validate('');

medium

A. null

B. '' (empty string)

C. undefined

D. 'Field cannot be empty'

Solution

1. Step 1: Analyze the input to validate function

   The input is an empty string ''. The trim() method removes spaces but here input is already empty.

2. Step 2: Follow the if condition logic

   Since text.trim() === '', the function returns 'Field cannot be empty'.

3. Final Answer:

   'Field cannot be empty' -> Option D

4. Quick Check:

   Empty input triggers error message [OK]

Hint: Empty strings trigger error messages in validation [OK]

Common Mistakes:

• Assuming empty string returns empty error
• Ignoring trim() effect
• Confusing return values

4. Identify the error in this validation function:

const validateEmail = (email) => {
  if (!email.includes('@')) {
    'Invalid email';
  }
  return '';
};

medium

A. Missing return statement inside if block

B. Using includes() is incorrect for email validation

C. Function should be async

D. Return statement should be outside the function

Solution

1. Step 1: Check if error message is returned

   The if block has a string 'Invalid email' but no return keyword, so it does nothing.

2. Step 2: Understand function flow

   Because no return in if, function always returns empty string, ignoring errors.

3. Final Answer:

   Missing return statement inside if block -> Option A

4. Quick Check:

   Return missing in if block causes no error shown [OK]

Hint: Always return error messages inside conditions [OK]

Common Mistakes:

• Forgetting return keyword in if block
• Assuming string alone returns value
• Misusing async when not needed

5. You want to validate a password field that must be at least 8 characters and contain a number. Which validation function correctly implements this?

hard

A. const validatePassword = (pwd) => pwd.length >= 8 || /\d/.test(pwd) ? '' : 'Password must be 8+ chars and include a number';

B. const validatePassword = (pwd) => { if (pwd.length < 8) return 'Too short'; if (!/\d/.test(pwd)) return 'No number'; return ''; };

C. const validatePassword = (pwd) => { if (pwd.length > 8 || /\d/.test(pwd)) return ''; else return 'Invalid'; };

D. const validatePassword = (pwd) => pwd.length > 8 && /\d/.test(pwd) ? 'Valid' : 'Invalid';

Solution

1. Step 1: Check each condition separately

   const validatePassword = (pwd) => { if (pwd.length < 8) return 'Too short'; if (!/\d/.test(pwd)) return 'No number'; return ''; }; checks length first, returns 'Too short' if less than 8, then checks for number, returns 'No number' if missing.

2. Step 2: Confirm final return for valid password

   If both conditions pass, it returns empty string meaning no error, which is correct.

3. Final Answer:

   const validatePassword = (pwd) => { if (pwd.length < 8) return 'Too short'; if (!/\d/.test(pwd)) return 'No number'; return ''; }; -> Option B

4. Quick Check:

   Separate checks with returns for each error = const validatePassword = (pwd) => { if (pwd.length < 8) return 'Too short'; if (!/\d/.test(pwd)) return 'No number'; return ''; }; [OK]

Hint: Check each rule separately and return specific errors [OK]

Common Mistakes:

• Using OR instead of AND for conditions
• Returning success message instead of empty string
• Combining conditions incorrectly