Request validation in Node.js

Introduction

Request validation helps check if the data sent to your server is correct and safe before using it.

When a user submits a form with their name and email.

When an API receives data from another app and needs to check it.

When you want to make sure numbers or dates sent are in the right format.

When you want to avoid errors caused by missing or wrong data.

When you want to protect your app from bad or harmful input.

Syntax

Node.js

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

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

app.post('/route', [
  body('fieldName').validationMethod(),
  // more validations
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  // handle valid data
  res.send('Success');
});

Use express-validator package to add validation rules easily.

Always check validationResult to find errors before processing data.

Examples

Checks if the 'email' field contains a valid email address.

Node.js

body('email').isEmail()

Checks if the 'age' field is an integer and at least 18.

Node.js

body('age').isInt({ min: 18 })

Checks if the 'password' field has at least 6 characters.

Node.js

body('password').isLength({ min: 6 })

Checks if the 'username' field is not empty.

Node.js

body('username').notEmpty()

Sample Program

This code creates a simple server that checks if the username is not empty, the email is valid, and the password is at least 6 characters long before accepting the registration.

Node.js

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

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

app.post('/register', [
  body('username').notEmpty(),
  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('User registered successfully');
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Always validate user input to avoid crashes and security issues.

Use clear error messages to help users fix their input.

Validation runs before your main code, so bad data never reaches your logic.

Summary

Request validation checks data before using it.

Use express-validator for easy rules in Node.js.

Always handle validation errors to keep your app safe and stable.

Practice

1. What is the main purpose of request validation in a Node.js app?

easy

A. To store data in the database

B. To speed up the server response time

C. To check incoming data before using it

D. To create new routes automatically

Solution

1. Step 1: Understand request validation

   Request validation means checking the data sent by users before the app uses it.

2. Step 2: Identify the main goal

   The goal is to ensure data is safe and correct to avoid errors or security issues.

3. Final Answer:

   To check incoming data before using it -> Option C

4. Quick Check:

   Request validation = check data first [OK]

Hint: Validation means checking data before use [OK]

Common Mistakes:

• Thinking validation speeds up server
• Confusing validation with data storage
• Assuming validation creates routes

2. Which code snippet correctly uses express-validator to check if 'email' is valid?

easy

A. validateEmail('email').isValid()

B. validate('email').isEmail()

C. checkEmail('email').validate()

D. check('email').isEmail()

Solution

1. Step 1: Recall express-validator syntax

   express-validator uses check('fieldName').validationMethod() to validate fields.

2. Step 2: Match correct method for email

   The method is isEmail() to check if the input is a valid email.

3. Final Answer:

   check('email').isEmail() -> Option D

4. Quick Check:

   express-validator uses check() + isEmail() [OK]

Hint: Use check('field').isEmail() for email validation [OK]

Common Mistakes:

• Using validate() instead of check()
• Wrong method names like validateEmail()
• Mixing method order or names

3. Given this code, what will be the output if the request body has { email: 'not-an-email' }?

app.post('/signup', [
  check('email').isEmail(),
  (req, res) => {
    const errors = validationResult(req);
    if (!errors.isEmpty()) {
      return res.status(400).json({ errors: errors.array() });
    }
    res.send('Success');
  }
]);

medium

A. Response with status 400 and error details

B. Response with 'Success' message

C. Server crashes with an error

D. Response with status 200 and empty body

Solution

1. Step 1: Check validation rule

   The rule check('email').isEmail() requires a valid email format.

2. Step 2: Analyze input and error handling

   The input 'not-an-email' fails isEmail(), so errors.isEmpty() is false, triggering status 400 with error details.

3. Final Answer:

   Response with status 400 and error details -> Option A

4. Quick Check:

   Invalid email triggers 400 error [OK]

Hint: Invalid input triggers 400 error with details [OK]

Common Mistakes:

• Assuming success response on invalid input
• Expecting server crash on validation fail
• Ignoring error handling logic

4. Identify the error in this validation middleware:

app.post('/login', [
  check('password').isLength({ min: 6 }),
  (req, res) => {
    const errors = validationResult;
    if (!errors.isEmpty()) {
      return res.status(422).json({ errors: errors.array() });
    }
    res.send('Logged in');
  }
]);

medium

A. validationResult is not called with req, so errors is not an object with isEmpty()

B. isLength should be isMinLength for password check

C. Missing next() call to continue middleware chain

D. Response status 422 is invalid for validation errors

Solution

1. Step 1: Check usage of validationResult

   validationResult must be called as validationResult(req) to get errors from the request.

2. Step 2: Identify the mistake in code

   The code assigns errors = validationResult without calling it, so errors is not an object with isEmpty().

3. Final Answer:

   validationResult is not called with req, so errors is not an object with isEmpty() -> Option A

4. Quick Check:

   Call validationResult(req) to get errors [OK]

Hint: Always call validationResult(req) to get errors [OK]

Common Mistakes:

• Forgetting to call validationResult as a function
• Using wrong validation method names
• Confusing middleware flow with next()

5. You want to validate a request body with fields 'username' (required, min 3 chars) and 'age' (optional, must be a number if present). Which express-validator setup is correct?

hard

A. [check('username').optional().isLength({ min: 3 }), check('age').isNumeric()]

B. [check('username').isLength({ min: 3 }).notEmpty(), check('age').optional().isNumeric()]

C. [check('username').isLength({ min: 3 }), check('age').notEmpty().isNumeric()]

D. [check('username').notEmpty(), check('age').optional().isString()]

Solution

1. Step 1: Validate 'username' requirements

   Username is required and must have at least 3 characters, so use notEmpty() and isLength({ min: 3 }).

2. Step 2: Validate 'age' requirements

   Age is optional but if present must be numeric, so use optional() and isNumeric().

3. Final Answer:

   [check('username').isLength({ min: 3 }).notEmpty(), check('age').optional().isNumeric()] -> Option B

4. Quick Check:

   Required username + optional numeric age = [check('username').isLength({ min: 3 }).notEmpty(), check('age').optional().isNumeric()] [OK]

Hint: Use optional() for optional fields, notEmpty() for required [OK]

Common Mistakes:

• Marking required fields as optional
• Not chaining optional() before other checks
• Using wrong validators like isString() for numbers