Manual validation patterns in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is manual validation in Express?

Manual validation means checking user input yourself in your code before using it. You write simple if-else checks to make sure data is correct and safe.

beginner

Why use manual validation instead of automatic libraries?

Manual validation gives you full control and helps you understand exactly what checks happen. It is simple for small apps and teaches core concepts.

beginner

Show a simple manual validation example for checking if a username exists and is a string.

if (!req.body.username || typeof req.body.username !== 'string') { res.status(400).send('Username is required and must be a string'); return; }

beginner

What is a common pattern to handle validation errors manually in Express?

Check inputs early in the route handler. If invalid, send a 400 response with an error message and stop further processing.

intermediate

How can manual validation improve security in Express apps?

By checking inputs carefully, you prevent bad or harmful data from reaching your app logic or database. This stops bugs and attacks like injection.

What should you do first in manual validation in Express?

A Check user input for required fields and types

B Send data to the database

C Render the response page

D Ignore errors and continue

Manual validation starts by checking if the input data exists and is the right type before using it.

If manual validation fails, what is the best response status code to send?

A 400 Bad Request

B 200 OK

C 500 Internal Server Error

D 302 Redirect

400 Bad Request tells the client their input was invalid, which is correct for validation errors.

Which of these is NOT a good manual validation check?

A Check if a field is the correct data type

B Check if the server has enough memory

C Check if a required field is missing

D Check if a string is not empty

Server memory is unrelated to manual validation of user input.

What happens if you skip manual validation in Express?

A User input is always correct

B App runs faster and safer

C App may crash or behave unexpectedly

D Database automatically cleans input

Skipping validation risks bad data causing errors or security issues.

Manual validation is best suited for:

A Apps that only use databases

B Huge apps with complex rules only

C Apps that never get user input

D Small apps or learning purposes

Manual validation is simple and good for small projects or to learn validation basics.

Explain how to manually validate a user registration form in Express.

Think about checking each input before saving or responding.

Describe the benefits and limitations of manual validation patterns in Express.

Consider when manual validation is helpful and when it might be better to use libraries.

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