Building custom middleware in Node.js

Introduction

Middleware helps you add extra steps when your server handles requests. It lets you run code before sending a response.

You want to check if a user is logged in before showing a page.

You need to log details about every request your server gets.

You want to add special headers to all responses.

You want to handle errors in one place for all requests.

Syntax

Node.js

function middlewareName(req, res, next) {
  // Your code here
  next();
}

The req is the request object with info about the client request.

The res is the response object used to send data back.

The next is a function you call to move to the next middleware or route.

Examples

This middleware logs the HTTP method and URL of each request.

Node.js

function logger(req, res, next) {
  console.log(`${req.method} ${req.url}`);
  next();
}

This middleware adds a custom header to every response.

Node.js

function addPoweredBy(req, res, next) {
  res.setHeader('X-Powered-By', 'Node.js');
  next();
}

This middleware checks if the request has an authorization header and blocks access if missing.

Node.js

function checkAuth(req, res, next) {
  if (!req.headers.authorization) {
    res.status(401).send('Unauthorized');
  } else {
    next();
  }
}

Sample Program

This example creates a simple Express server. It uses a custom middleware called logger that prints the HTTP method and URL for every request. Then it responds with 'Hello, world!' on the home page.

Node.js

import express from 'express';

const app = express();

// Custom middleware to log requests
function logger(req, res, next) {
  console.log(`${req.method} ${req.url}`);
  next();
}

// Use the middleware for all routes
app.use(logger);

app.get('/', (req, res) => {
  res.send('Hello, world!');
});

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

Important Notes

Always call next() in your middleware unless you send a response directly.

Middleware runs in the order you add it with app.use() or on routes.

You can create middleware to handle errors by defining a function with four arguments: (err, req, res, next).

Summary

Middleware lets you add extra steps when handling requests.

It uses three arguments: req, res, and next.

Call next() to continue to the next step or send a response to end.

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