Manual validation patterns in Express - Mini Project: Build & Apply

Manual validation patterns in Express

📖 Scenario: You are building a simple Express server that accepts user registration data. You want to manually check the data before saving it.

🎯 Goal: Create an Express route that manually validates the username and age fields from the request body. If validation passes, respond with success; otherwise, respond with an error message.

📋 What You'll Learn

Create an Express app with a POST route at /register

Manually check that username exists and is a string

Manually check that age exists and is a number greater than or equal to 18

Send a JSON response with { success: true } if valid

Send a JSON response with { success: false, error: 'message' } if invalid

💡 Why This Matters

🌍 Real World

Manual validation is common in backend servers to ensure data is correct before processing or saving.

💼 Career

Backend developers often write manual validation logic in Express to handle user input safely and provide clear error messages.

1

Set up Express app and import modules

Write code to import express, create an app with express(), and use express.json() middleware.

Express

const express = require('express');
// Your code here

Hint

Use require('express') to import Express. Create the app by calling express(). Use app.use(express.json()) to parse JSON bodies.

2

Create POST route at /register

Add a POST route handler on app for path '/register' with parameters req and res.

Express

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

// Your code here

Hint

Use app.post('/register', (req, res) => { ... }) to create the route.

3

Manually validate username and age

Inside the /register route, extract username and age from req.body. Check if username is a string and exists, and age is a number >= 18. If invalid, respond with res.json({ success: false, error: 'Invalid input' }) and return.

Express

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

app.post('/register', (req, res) => {
  // Your code here
});

Hint

Use const { username, age } = req.body to get data. Check types with typeof. Use return res.json(...) to send error and stop.

4

Send success response and start server

If validation passes, respond with res.json({ success: true }). Then add app.listen(3000) to start the server on port 3000.

Express

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

app.post('/register', (req, res) => {
  const { username, age } = req.body;
  if (typeof username !== 'string' || !username) {
    return res.json({ success: false, error: 'Invalid input' });
  }
  if (typeof age !== 'number' || age < 18) {
    return res.json({ success: false, error: 'Invalid input' });
  }
  // Your code here
});

// Your code here

Hint

Send success with res.json({ success: true }). Start server with app.listen(3000).

Practice

1. What is the main purpose of manual validation in Express route handlers?

easy

A. To check user input step-by-step and catch bad data early

B. To automatically generate database schemas

C. To speed up server response time by skipping checks

D. To style the user interface dynamically

Solution

1. Step 1: Understand manual validation role

   Manual validation means checking user input carefully in your code before using it.

2. Step 2: Identify the main goal

   The goal is to catch bad or incorrect data early to keep the app safe and user-friendly.

3. Final Answer:

   To check user input step-by-step and catch bad data early -> Option A

4. Quick Check:

   Manual validation = catch bad data early [OK]

Hint: Manual validation means checking input carefully yourself [OK]

Common Mistakes:

• Thinking validation auto-generates database code
• Believing validation speeds up server by skipping checks
• Confusing validation with UI styling

2. Which of the following is the correct way to manually validate that a request body has a non-empty 'username' field in Express?

easy

A. if (req.body.username === undefined) { next(); }

B. if (!req.body.username) { res.status(400).send('Username required'); }

C. if (req.body.username.length === 0) { res.sendStatus(200); }

D. if (req.body.username == null) { res.redirect('/'); }

Solution

1. Step 1: Check for missing or empty username

   Using !req.body.username checks if username is missing or empty string.

2. Step 2: Respond with error status and message

   Sending status 400 with message 'Username required' correctly informs client of bad input.

3. Final Answer:

   if (!req.body.username) { res.status(400).send('Username required'); } -> Option B

4. Quick Check:

   Check missing username and send 400 error [OK]

Hint: Use if (!field) to check missing or empty string [OK]

Common Mistakes:

• Using next() instead of sending error response
• Sending 200 OK on invalid input
• Redirecting instead of responding with error

3. Consider this Express route snippet:
<pre>app.post('/submit', (req, res) => { if (typeof req.body.age !== 'number' || req.body.age < 18) { return res.status(400).send('Age must be 18 or older'); } res.send('Welcome!'); });
What will be the response if the client sends {"age": 16} in JSON body?

medium

A. Status 500 server error

B. Status 200 with message 'Welcome!'

C. Status 400 with message 'Age must be 18 or older'

D. No response, request hangs

Solution

1. Step 1: Check age type and value

   The code checks if age is not a number or less than 18. Here age is 16, a number but less than 18.

2. Step 2: Return 400 error with message

   Since age < 18, the code returns status 400 with message 'Age must be 18 or older'.

3. Final Answer:

   Status 400 with message 'Age must be 18 or older' -> Option C

4. Quick Check:

   Age 16 triggers 400 error [OK]

Hint: Check conditions carefully to predict response status [OK]

Common Mistakes:

• Assuming 16 passes validation
• Expecting 200 OK instead of error
• Thinking server crashes on invalid input

4. Identify the bug in this manual validation code snippet:

app.post('/login', (req, res) => {
  if (req.body.password.length < 8) {
    res.status(400).send('Password too short');
  }
  res.send('Login successful');
});

medium

A. Missing return after sending error response causes double response

B. Password length check should be > 8, not < 8

C. Should use req.query instead of req.body

D. res.send should be res.json for JSON response

Solution

1. Step 1: Analyze error response flow

   The code sends error response if password is too short but does not stop execution.

2. Step 2: Identify missing return causes double response

   Without return, the code continues and sends 'Login successful' response, causing error.

3. Final Answer:

   Missing return after sending error response causes double response -> Option A

4. Quick Check:

   Return after error response to stop execution [OK]

Hint: Always return after sending error response to avoid double send [OK]

Common Mistakes:

• Thinking length check direction is wrong
• Confusing req.body with req.query
• Believing res.send must be res.json

5. You want to manually validate a user registration form in Express. The form requires 'email' (non-empty string), 'password' (min 8 chars), and 'age' (optional, but if present must be number >= 13). Which code snippet correctly implements this validation?

hard

A. if (!req.body.email || req.body.email.length === 0) {
return res.status(400).send('Email required');
}
if (req.body.password.length < 8) {
return res.status(200).send('Password too short');
}
if (req.body.age && typeof req.body.age !== 'string') {
return res.status(400).send('Age must be a string');
}
next();

B. if (!req.body.email) {
res.send('Email missing');
}
if (req.body.password.length <= 8) {
res.send('Password invalid');
}
if (req.body.age < 13) {
res.send('Too young');
}
next();

C. if (req.body.email === '') {
return res.status(500).send('Email error');
}
if (req.body.password.length > 8) {
return res.status(400).send('Password too short');
}
if (req.body.age && req.body.age < 13) {
return res.status(400).send('Age error');
}
next();

D. if (!req.body.email || typeof req.body.email !== 'string') {
return res.status(400).send('Email required');
}
if (!req.body.password || req.body.password.length < 8) {
return res.status(400).send('Password too short');
}
if (req.body.age !== undefined && (typeof req.body.age !== 'number' || req.body.age < 13)) {
return res.status(400).send('Age must be 13 or older');
}
next();

Solution

1. Step 1: Validate email presence and type

   if (!req.body.email || typeof req.body.email !== 'string') checks for missing, empty, or non-string email and returns 400 error if invalid.

2. Step 2: Validate password length correctly

   if (!req.body.password || req.body.password.length < 8) checks for missing or short password (<8 chars) and returns 400 error.

3. Step 3: Validate optional age correctly

   if (req.body.age !== undefined && (typeof req.body.age !== 'number' || req.body.age < 13)) checks if age provided, then ensures it's a number >=13, returns 400 if invalid.

4. Final Answer:

   if (!req.body.email || typeof req.body.email !== 'string') { return res.status(400).send('Email required'); } if (!req.body.password || req.body.password.length < 8) { return res.status(400).send('Password too short'); } if (req.body.age !== undefined && (typeof req.body.age !== 'number' || req.body.age < 13)) { return res.status(400).send('Age must be 13 or older'); } next(); -> Option D

5. Quick Check:

   All fields validated with correct conditions and error codes [OK]

Hint: Check each field with proper type and conditions, return on error [OK]

Common Mistakes:

• Not returning after sending error response
• Using wrong status codes like 200 or 500 for validation errors
• Checking wrong types or missing optional field checks