express-validator setup - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is express-validator used for in an Express app?

express-validator helps check and validate user input in Express apps to keep data clean and safe.

beginner

How do you install express-validator in your project?

Run npm install express-validator in your project folder to add it.

beginner

Which function from express-validator do you use to define validation rules?

You use check() or body() to set rules for request data fields.

intermediate

What does validationResult(req) do in your route handler?

It collects the results of validations and tells you if there are errors in the user input.

intermediate

Show a simple example of setting up express-validator in an Express POST route.

<pre>import { check, validationResult } from 'express-validator';

app.post('/signup', [
  check('email').isEmail(),
  check('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');
});</pre>

What command installs express-validator?

A npm install express

B npm install express-validator

C npm install validator

D npm install express-validator-cli

The correct package is express-validator, installed with npm install express-validator.

Which function is used to check a field in express-validator?

A check()

B verify()

C validate()

D inspect()

check() is used to define validation rules for request fields.

What does validationResult(req) return?

A The original request object

B A boolean true if valid

C An array of validation errors

D The response object

It returns an object containing any validation errors found in the request.

Where do you place validation middleware in an Express route?

A In the app.listen callback

B After the route handler

C Inside the route handler only

D Before the route handler

Validation middleware runs before the route handler to check input first.

Which of these is a valid validation rule with express-validator?

A check('email').isEmail()

B check('name').isBoolean()

C check('password').isNumber()

D check('age').isEmail()

check('email').isEmail() correctly checks if the email field is a valid email.

Explain how to set up express-validator in an Express POST route to validate user input.

Think about middleware placement and error checking.

Describe the role of validationResult in the validation process with express-validator.

It helps decide if input is good or not.

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