express-validator setup - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this express-validator middleware?

Consider this Express route using express-validator. What will be the response if the request body is { "email": "invalidemail", "age": 17 }?

Express

import express from 'express';
import { body, validationResult } from 'express-validator';

const app = express();
app.use(express.json());

app.post('/register', [
  body('email').isEmail(),
  body('age').isInt({ min: 18 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.json({ message: 'User registered' });
});

A Status 200 with message 'User registered'

B Status 400 with errors array containing email and age validation errors

C Status 400 with errors array containing only email validation error

D Status 400 with errors array containing only age validation error

💡 Hint

Check which validations fail for the given input values.

The email is invalid and age is less than 18, so both validations fail. The response returns status 400 with both errors.

📝 Syntax

intermediate

Which option correctly imports and uses express-validator in an Express app?

Select the option that correctly sets up express-validator middleware for validating a username field as non-empty.

A

import { body } from 'express-validator';
app.post('/user', [body('username').notEmpty()], handler);

B

const { validate } = require('express-validator');
app.post('/user', [validate('username').notEmpty()], handler);

C

import validator from 'express-validator';
app.post('/user', [validator('username').notEmpty()], handler);

D

import { check } from 'express-validator';
app.post('/user', [check('username').notEmpty()], handler);

💡 Hint

Check the official express-validator import and method names.

Option A correctly imports 'body' from express-validator and uses it to validate the 'username' field.

🔧 Debug

advanced

Why does this express-validator code not catch validation errors?

Given this code snippet, why does the server always respond with 'User created' even if the input is invalid? app.post('/create', [body('email').isEmail()], (req, res) => { res.send('User created'); });

A Because body() middleware is not imported correctly

B Because isEmail() does not validate email format

C Because express.json() middleware is missing

D Because validationResult is not called to check for errors before sending response

💡 Hint

Think about how express-validator reports validation errors.

express-validator requires calling validationResult(req) to check for errors. Without this, errors are ignored.

❓ state_output

advanced

What is the value of errors after this validation?

Given this code snippet, what will be the value of errors.array() if the request body is { "password": "123" }? app.post('/signup', [ body('password').isLength({ min: 6 }) ], (req, res) => { const errors = validationResult(req); res.json({ errors: errors.array() }); });

A [{ msg: 'Password must be at least 6 characters', param: 'password', location: 'body' }]

B []

C [{ msg: 'Invalid value', param: 'password', location: 'body' }]

D null

💡 Hint

Check the default error message for isLength validator.

The default error message for isLength is 'Invalid value' when the length condition fails.

🧠 Conceptual

expert

Which statement about express-validator setup is true?

Choose the correct statement about how express-validator integrates with Express routes.

A validationResult(req) must be called inside the route handler to check for validation errors

B express-validator middleware must be placed before express.json() to parse request body

C express-validator automatically sends error responses if validation fails

D Validation chains can only be used globally, not per route

💡 Hint

Think about how validation errors are handled in express-validator.

express-validator requires explicit call to validationResult(req) inside route handler to check errors. It does not send responses automatically.

Practice

1. What is the main purpose of using express-validator in an Express app?

easy

A. To handle server errors automatically

B. To create database connections

C. To serve static files

D. To check and clean user input data

Solution

1. Step 1: Understand express-validator's role

   express-validator is a tool used to validate and sanitize user input in Express applications.

2. Step 2: Compare options with express-validator's purpose

   Only 'To check and clean user input data' matches this purpose. Options A, B, and C relate to other Express features.

3. Final Answer:

   To check and clean user input data -> Option D

4. Quick Check:

   express-validator = input validation [OK]

Hint: express-validator is for input checks, not server or DB tasks [OK]

Common Mistakes:

• Confusing express-validator with database tools
• Thinking it handles static files or errors automatically

2. Which of the following is the correct way to import the body validator from express-validator?

easy

A. const { body } = require('express-validator');

B. import { body } from 'express-validator';

C. import body from 'express-validator';

D. const body = require('express-validator').body();

Solution

1. Step 1: Identify modern import syntax for express-validator

   express-validator exports named functions like body, so use named import syntax.

2. Step 2: Choose correct ES module import

   import { body } from 'express-validator'; uses import { body } from 'express-validator'; which is correct for ES modules.

3. Final Answer:

   import { body } from 'express-validator'; -> Option B

4. Quick Check:

   Named import syntax = import { body } from 'express-validator'; [OK]

Hint: Use named imports with curly braces for express-validator [OK]

Common Mistakes:

• Using default import instead of named import
• Calling body() during import
• Using require without destructuring

3. Given this Express route setup using express-validator, what will be the response if the email field is missing in the request body?

import { body, validationResult } from 'express-validator';

app.post('/signup', [
  body('email').isEmail(),
  body('password').isLength({ min: 6 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.send('Signup successful');
});

medium

A. Status 400 with error about missing or invalid email

B. Status 200 with message 'Signup successful'

C. Status 500 server error

D. Status 400 with error about password length only

Solution

1. Step 1: Understand validation rules

   The route requires 'email' to be a valid email and 'password' to be at least 6 characters.

2. Step 2: Analyze missing email field effect

   Missing 'email' fails isEmail() check, so validationResult(req) will contain errors.

3. Step 3: Check response on validation failure

   The code returns status 400 with error details if errors exist.

4. Final Answer:

   Status 400 with error about missing or invalid email -> Option A

5. Quick Check:

   Missing email triggers validation error = Status 400 with error about missing or invalid email [OK]

Hint: Missing required field triggers 400 error with validation message [OK]

Common Mistakes:

• Assuming success response despite missing fields
• Confusing status codes for validation errors
• Ignoring validationResult check

4. Identify the error in this express-validator setup:

import { body, validationResult } from 'express-validator';

app.post('/login', (req, res) => {
  body('username').notEmpty();
  body('password').isLength({ min: 8 });

  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(422).json({ errors: errors.array() });
  }
  res.send('Login successful');
});

medium

A. Validators are not used as middleware before the route handler

B. validationResult is called incorrectly

C. Missing import of express

D. Response status code 422 is invalid

Solution

1. Step 1: Check how validators are applied

   Validators like body('username').notEmpty() must be middleware before the route handler, not called inside it.

2. Step 2: Identify correct middleware usage

   Validators should be passed as an array before the handler function in app.post.

3. Final Answer:

   Validators are not used as middleware before the route handler -> Option A

4. Quick Check:

   Validators must be middleware, not called inside handler [OK]

Hint: Validators go before handler as middleware, not inside it [OK]

Common Mistakes:

• Calling validators inside route handler function
• Ignoring middleware order
• Assuming validationResult usage is wrong

5. You want to validate a user registration form with fields: email, password, and age. The rules are: email must be valid, password at least 8 characters, and age must be an integer between 18 and 99. Which express-validator setup correctly applies these rules and handles errors?

hard

A. app.post('/register', [
body('email').isEmail(),
body('password').isLength({ min: 8 }),
body('age').isInt({ min: 18, max: 99 })
], (req, res) => {
res.send('Registration complete');
});

B. app.post('/register', (req, res) => {
body('email').isEmail();
body('password').isLength({ min: 8 });
body('age').isInt({ min: 18, max: 99 });
const errors = validationResult(req);
if (!errors.isEmpty()) {
return res.status(400).json({ errors: errors.array() });
}
res.send('Registration complete');
});

C. app.post('/register', [
body('email').isEmail(),
body('password').isLength({ min: 8 }),
body('age').isInt({ min: 18, max: 99 })
], (req, res) => {
const errors = validationResult(req);
if (!errors.isEmpty()) {
return res.status(400).json({ errors: errors.array() });
}
res.send('Registration complete');
});

D. app.post('/register', [
body('email').isEmail(),
body('password').isLength({ min: 8 }),
body('age').isInt({ min: 18, max: 99 })
], (req, res) => {
const errors = validationResult(req);
if (errors.isEmpty()) {
return res.status(400).json({ errors: errors.array() });
}
res.send('Registration complete');
});

Solution

1. Step 1: Check middleware usage for validators

   Validators must be passed as middleware array before the route handler, as in three of the choices.

2. Step 2: Verify error handling logic

   app.post('/register', [ body('email').isEmail(), body('password').isLength({ min: 8 }), body('age').isInt({ min: 18, max: 99 }) ], (req, res) => { const errors = validationResult(req); if (!errors.isEmpty()) { return res.status(400).json({ errors: errors.array() }); } res.send('Registration complete'); }); correctly checks !errors.isEmpty() to detect errors and respond with status 400. app.post('/register', [ body('email').isEmail(), body('password').isLength({ min: 8 }), body('age').isInt({ min: 18, max: 99 }) ], (req, res) => { res.send('Registration complete'); }); skips error checking. app.post('/register', [ body('email').isEmail(), body('password').isLength({ min: 8 }), body('age').isInt({ min: 18, max: 99 }) ], (req, res) => { const errors = validationResult(req); if (errors.isEmpty()) { return res.status(400).json({ errors: errors.array() }); } res.send('Registration complete'); }); reverses the condition incorrectly. app.post('/register', (req, res) => { body('email').isEmail(); body('password').isLength({ min: 8 }); body('age').isInt({ min: 18, max: 99 }); const errors = validationResult(req); if (!errors.isEmpty()) { return res.status(400).json({ errors: errors.array() }); } res.send('Registration complete'); }); calls validators inside handler, which is wrong.

3. Final Answer:

   The setup with validators as middleware array and correct !errors.isEmpty() check -> Option C

4. Quick Check:

   Middleware validators + correct error check = app.post('/register', [ body('email').isEmail(), body('password').isLength({ min: 8 }), body('age').isInt({ min: 18, max: 99 }) ], (req, res) => { const errors = validationResult(req); if (!errors.isEmpty()) { return res.status(400).json({ errors: errors.array() }); } res.send('Registration complete'); }); [OK]

Hint: Use middleware array and check !errors.isEmpty() for validation [OK]

Common Mistakes:

• Calling validators inside route handler
• Skipping validationResult error check
• Reversing error condition logic