Building custom middleware in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is middleware in Node.js Express?

Middleware is a function that runs during the request-response cycle. It can modify the request or response, or end the cycle by sending a response.

beginner

How do you define a custom middleware function in Express?

A custom middleware is a function with three parameters: req, res, and next. It processes the request and calls next() to pass control to the next middleware.

beginner

What does the next() function do in middleware?

next() passes control to the next middleware or route handler. Without calling next(), the request will hang and not proceed.

intermediate

How can middleware modify the request object?

Middleware can add new properties or change existing ones on req. For example, adding req.user after authentication.

intermediate

Where should you place custom middleware in your Express app?

Place middleware before the routes that need it. This ensures the middleware runs first and can affect those routes.

What are the three parameters of an Express middleware function?

A request, response, finish

B request, response, callback

C req, res, done

D req, res, next

Express middleware functions always receive req, res, and next as parameters.

What happens if you forget to call next() in your middleware?

A The server will crash immediately.

B The request will hang and never reach the next middleware or route.

C The response will automatically be sent.

D The middleware will skip to the last route.

Without calling next(), Express does not know to continue, so the request hangs.

Which of these is a common use case for custom middleware?

A Logging requests

B Rendering HTML templates

C Connecting to a database

D Serving static files

Logging requests is a typical middleware task to track incoming traffic.

Where should you place middleware in your Express app?

A Before the routes that need it

B After all routes

C Only inside route handlers

D Anywhere, order does not matter

Middleware must be placed before routes to affect those routes.

How can middleware add data to be used later in the request cycle?

A By modifying the <code>res</code> headers only

B By sending a response immediately

C By adding properties to the <code>req</code> object

D By calling <code>next()</code> twice

Middleware can add properties to req so later middleware or routes can use that data.

Explain how to create and use a custom middleware function in Express.

Think about the function signature and where it fits in the request flow.

Describe what happens if you do not call next() inside your middleware.

Consider the flow of control in Express middleware.

Practice

1. What is the main purpose of custom middleware in a Node.js Express app?

easy

A. To directly send HTML pages to the client

B. To add extra processing steps between receiving a request and sending a response

C. To replace the Express router completely

D. To store data permanently in a database

Solution

1. Step 1: Understand middleware role

   Middleware functions run between receiving a request and sending a response, allowing extra processing.

2. Step 2: Identify correct purpose

   Custom middleware adds steps like logging, authentication, or modifying requests before response.

3. Final Answer:

   To add extra processing steps between receiving a request and sending a response -> Option B

4. Quick Check:

   Middleware = extra processing steps [OK]

Hint: Middleware adds steps between request and response [OK]

Common Mistakes:

• Thinking middleware sends responses directly always
• Confusing middleware with routing
• Assuming middleware stores data permanently

2. Which of the following is the correct syntax to define a simple middleware function in Express?

easy

A. app.use = function(req, res, next) { console.log('Hi'); next(); }

B. app.middleware(req, res, next) { console.log('Hi'); next(); }

C. app.use(req, res, next) => { console.log('Hi'); next(); }

D. app.use((req, res, next) => { console.log('Hi'); next(); });

Solution

1. Step 1: Recall Express middleware syntax

   Middleware is added using app.use() with a function taking (req, res, next).

2. Step 2: Check each option

   app.use((req, res, next) => { console.log('Hi'); next(); }); correctly uses app.use with an arrow function and calls next(). Others have syntax errors.

3. Final Answer:

   app.use((req, res, next) => { console.log('Hi'); next(); }); -> Option D

4. Quick Check:

   app.use + (req, res, next) function = correct [OK]

Hint: Middleware uses app.use with (req, res, next) function [OK]

Common Mistakes:

• Missing parentheses around parameters
• Assigning app.use instead of calling it
• Using arrow function without parentheses for multiple params

3. Consider this middleware code snippet:

app.use((req, res, next) => {
  req.customValue = 42;
  next();
});
app.get('/', (req, res) => {
  res.send(`Value is ${req.customValue}`);
});

What will be the response when a client requests '/'?

medium

A. Value is 42

B. Value is undefined

C. Error: req.customValue is not defined

D. Empty response

Solution

1. Step 1: Understand middleware effect on req object

   The middleware adds a property customValue with 42 to the req object and calls next() to continue.

2. Step 2: Check route handler usage

   The route handler reads req.customValue and sends it in the response string.

3. Final Answer:

   Value is 42 -> Option A

4. Quick Check:

   Middleware sets req.customValue = 42 [OK]

Hint: Middleware can add properties to req used later [OK]

Common Mistakes:

• Assuming req.customValue is lost after middleware
• Thinking next() stops execution
• Expecting error without setting property

4. Identify the error in this middleware code:

app.use((req, res, next) => {
  if (!req.headers['x-auth']) {
    res.status(401).send('Unauthorized');
  }
  next();
});

medium

A. Calling next() after sending a response causes error

B. Missing call to next() inside the if block

C. Headers should be accessed as req.header, not req.headers

D. Middleware must always send a response

Solution

1. Step 1: Analyze middleware flow

   If 'x-auth' header is missing, response is sent with 401 status, ending the request.

2. Step 2: Check next() call after response

   Calling next() after sending a response causes Express to continue processing, leading to errors.

3. Final Answer:

   Calling next() after sending a response causes error -> Option A

4. Quick Check:

   Do not call next() after res.send [OK]

Hint: Don't call next() after sending response [OK]

Common Mistakes:

• Calling next() after res.send causing double response
• Assuming headers property name is wrong
• Thinking middleware must always send response

5. You want to create middleware that logs request method and URL only for POST requests. Which code correctly implements this?

hard

A. app.use((req, res, next) => { if (req.method = 'POST') console.log(req.method, req.url); next(); });

B. app.use((req, res, next) => { if (req.method === 'post') console.log(req.method, req.url); next(); });

C. app.use((req, res, next) => { if (req.method === 'POST') console.log(req.method, req.url); next(); });

D. app.use((req, res, next) => { if (req.method === POST) console.log(req.method, req.url); next(); });

Solution

1. Step 1: Check condition for POST method

   Correct comparison uses '===' and string 'POST' in uppercase.

2. Step 2: Verify logging and next call

   app.use((req, res, next) => { if (req.method === 'POST') console.log(req.method, req.url); next(); }); logs method and URL only if method is POST, then calls next() to continue.

3. Final Answer:

   app.use((req, res, next) => { if (req.method === 'POST') console.log(req.method, req.url); next(); }); -> Option C

4. Quick Check:

   Use strict equality and uppercase 'POST' [OK]

Hint: Use === 'POST' to check method string correctly [OK]

Common Mistakes:

• Using single = instead of === for comparison
• Using lowercase 'post' instead of uppercase
• Using undefined POST variable without quotes