Custom validation rules in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the express-validator check function.

Express

const { [1] } = require('express-validator');

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

Acheck

Bvalidate

Cverify

Dassert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'validate' or 'verify' instead of 'check'.

Forgetting to destructure the import.

The check function is imported from express-validator to define validation rules.

2fill in blank

medium

Complete the code to create a custom validator that checks if a value is an even number.

Express

check('number').custom(value => value [1] 2 === 0);

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

A===

B%

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '===' instead of '%'.

Using '+' or '-' which do not check divisibility.

The modulo operator % is used to check if the number is divisible by 2 with no remainder, meaning it is even.

3fill in blank

hard

Fix the error in the custom validator to correctly reject odd numbers with an error message.

Express

check('number').custom(value => {
  if (value [1] 2 !== 0) {
    throw new Error('Number must be even');
  }
  return true;
});

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

A+

B===

C%

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '===' instead of '%'.

Using '+' or '-' which do not check divisibility.

The modulo operator % is needed to check if the number is not divisible by 2.

4fill in blank

hard

Fill both blanks to create a custom validator that checks if a username starts with a capital letter and is at least 3 characters long.

Express

check('username').custom(value => {
  if (!(/^[2]/[1](value))) {
    throw new Error('Username must start with a capital letter');
  }
  if (value.length < 3) {
    throw new Error('Username must be at least 3 characters');
  }
  return true;
});

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

Amatch

Btest

CstartsWith

D[A-Z]

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'match' which returns an array instead of boolean.

Using 'startsWith' which does not accept regex.

Using incorrect regex pattern.

The test method checks if the string matches the regular expression /^[A-Z]/, which means it starts with a capital letter.

5fill in blank

hard

Fill all three blanks to create a custom validator that checks if an email ends with '.com' and contains '@', and throws an error if not.

Express

check('email').custom(value => {
  if (!value[1]('@')) {
    throw new Error('Email must contain @');
  }
  if (!(/\.[3]$/[2](value))) {
    throw new Error('Email must end with .com');
  }
  return true;
});

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

Aincludes

Btest

Ccom

Dmatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'match' instead of 'test' for regex boolean check.

Using 'startsWith' instead of 'includes' for '@'.

Incorrect regex pattern for '.com'.

The includes method checks if '@' is present. The test method checks if the string ends with '.com' using the regex /\.com$/.

Practice

1. What is the main purpose of using custom() in Express validation?

easy

A. To format the response JSON

B. To automatically sanitize all inputs

C. To connect to the database

D. To create your own rules for checking input values

Solution

1. Step 1: Understand the role of custom()

   The custom() method allows you to write your own validation logic beyond built-in checks.

2. Step 2: Identify the purpose in input validation

   It is used to check inputs with rules you define, like checking a password strength or a special format.

3. Final Answer:

   To create your own rules for checking input values -> Option D

4. Quick Check:

   Custom validation = custom rules [OK]

Hint: Custom means you write your own check function [OK]

Common Mistakes:

• Thinking custom() sanitizes inputs automatically
• Confusing custom() with response formatting
• Assuming custom() connects to databases

2. Which of the following is the correct syntax to add a custom validation rule using Express Validator?

easy

A. check('age').custom(value => { if(value < 18) throw new Error('Too young'); return true; })

B. check('age').custom(value => value < 18 ? true : false)

C. check('age').custom(value => { return false; })

D. check('age').custom(value => { throw 'Error'; })

Solution

1. Step 1: Review correct custom validation syntax

   The function inside custom() should throw an error if validation fails and return true if it passes.

2. Step 2: Analyze each option

   check('age').custom(value => { if(value < 18) throw new Error('Too young'); return true; }) throws an error when value is less than 18 and returns true otherwise, which is correct. check('age').custom(value => value < 18 ? true : false) returns true when value is less than 18, which is opposite logic. check('age').custom(value => { return false; }) always returns false, which fails validation. check('age').custom(value => { throw 'Error'; }) throws an error unconditionally, so it always fails.

3. Final Answer:

   check('age').custom(value => { if(value < 18) throw new Error('Too young'); return true; }) -> Option A

4. Quick Check:

   Throw error on fail, return true on pass [OK]

Hint: Throw error to fail, return true to pass [OK]

Common Mistakes:

• Returning false instead of throwing error
• Throwing error without condition
• Returning true on invalid input

3. Given this code snippet, what will be the validation result if req.body.username is "abc"?

check('username').custom(value => {
  if(value.length < 5) throw new Error('Too short');
  return true;
})

medium

A. Validation fails with 'Too short' error

B. Validation passes

C. Validation fails with syntax error

D. Validation passes but logs a warning

Solution

1. Step 1: Check the input value length

   The input "abc" has length 3, which is less than 5.

2. Step 2: Apply the custom validation logic

   The function throws an error 'Too short' if length is less than 5, so it throws an error here causing validation to fail.

3. Final Answer:

   Validation fails with 'Too short' error -> Option A

4. Quick Check:

   Input too short = error thrown [OK]

Hint: Check input length against condition in custom() [OK]

Common Mistakes:

• Assuming validation passes for short input
• Confusing error throwing with warnings
• Expecting syntax errors from valid code

4. Identify the error in this custom validation code:

check('email').custom(value => {
  if(!value.includes('@'))
    return new Error('Invalid email');
  return true;
})

medium

A. The function must return false instead of true

B. The condition should check for '.' instead of '@'

C. It should throw an error, not return it

D. No error, code is correct

Solution

1. Step 1: Understand error signaling in custom validation

   Custom validators must throw an error to indicate failure, not return an Error object.

2. Step 2: Analyze the given code

   The code returns new Error('Invalid email') instead of throwing it, so validation will not fail as expected.

3. Final Answer:

   It should throw an error, not return it -> Option C

4. Quick Check:

   Throw error to fail validation [OK]

Hint: Throw errors, don't return them in custom() [OK]

Common Mistakes:

• Returning Error object instead of throwing
• Checking wrong condition for email
• Returning false instead of throwing error

5. You want to create a custom validation rule that checks if a password contains at least one uppercase letter, one number, and is at least 8 characters long. Which of these implementations correctly achieves this?

hard

A. check('password').custom(value => {
if(value.length < 8) return false;
if(!/[A-Z]/.test(value)) return false;
if(!/\d/.test(value)) return false;
return true;
})

B. check('password').custom(value => {
if(!/[A-Z]/.test(value)) throw new Error('Missing uppercase');
if(!/\d/.test(value)) throw new Error('Missing number');
if(value.length < 8) throw new Error('Too short');
return true;
})

C. check('password').custom(value => {
if(value.length < 8) throw 'Too short';
if(!/[A-Z]/.test(value)) throw 'Missing uppercase';
if(!/\d/.test(value)) throw 'Missing number';
return false;
})

D. check('password').custom(value => {
if(value.length >= 8 && /[A-Z]/.test(value) && /\d/.test(value)) return true;
else return false;
})

Solution

1. Step 1: Check each condition with proper error throwing

   check('password').custom(value => { if(!/[A-Z]/.test(value)) throw new Error('Missing uppercase'); if(!/\d/.test(value)) throw new Error('Missing number'); if(value.length < 8) throw new Error('Too short'); return true; }) checks each condition separately and throws a specific error if it fails, returning true only if all pass.

2. Step 2: Compare other options for correctness

   check('password').custom(value => { if(value.length < 8) return false; if(!/[A-Z]/.test(value)) return false; if(!/\d/.test(value)) return false; return true; }) returns false instead of throwing errors, which is incorrect. check('password').custom(value => { if(value.length < 8) throw 'Too short'; if(!/[A-Z]/.test(value)) throw 'Missing uppercase'; if(!/\d/.test(value)) throw 'Missing number'; return false; }) throws string errors and returns false at the end, which breaks the rule of returning true on success. check('password').custom(value => { if(value.length >= 8 && /[A-Z]/.test(value) && /\d/.test(value)) return true; else return false; }) returns false instead of throwing an error if conditions fail, which is incorrect.

3. Final Answer:

   check('password').custom(value => { if(!/[A-Z]/.test(value)) throw new Error('Missing uppercase'); if(!/\d/.test(value)) throw new Error('Missing number'); if(value.length < 8) throw new Error('Too short'); return true; }) -> Option B

4. Quick Check:

   Throw specific errors, return true if all pass [OK]

Hint: Throw specific errors for each fail, return true if all pass [OK]

Common Mistakes:

• Returning false instead of throwing errors
• Throwing strings instead of Error objects
• Returning false on success