Building custom middleware in Node.js - Performance & Optimization

Performance: Building custom middleware

MEDIUM IMPACT

This affects server response time and how quickly the server processes requests before sending responses.

Processing HTTP requests with custom middleware in Node.js

Node.js

app.use(async (req, res, next) => {
  await someAsyncOperation();
  next();
});

Using asynchronous operations prevents blocking the event loop, allowing other requests to be handled concurrently.

📈 Performance GainNon-blocking middleware improves throughput and reduces response delays.

Processing HTTP requests with custom middleware in Node.js

Node.js

app.use((req, res, next) => {
  // heavy synchronous computation
  for (let i = 0; i < 1e8; i++) {}
  next();
});

Blocking the event loop with heavy synchronous code delays all requests, increasing response time.

📉 Performance CostBlocks event loop for hundreds of milliseconds, increasing server response time and INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Blocking synchronous middleware	0 (server-side)	0 (server-side)	0 (server-side)	[X] Bad
Asynchronous non-blocking middleware	0 (server-side)	0 (server-side)	0 (server-side)	[OK] Good

Rendering Pipeline

Middleware runs on the server before sending responses, affecting how fast the server can start sending data to the browser.

→Request Handling

→Response Preparation

⚠️ BottleneckBlocking synchronous code in middleware delays response start time.

Core Web Vital Affected

INP

This affects server response time and how quickly the server processes requests before sending responses.

Optimization Tips

1Avoid heavy synchronous code in middleware to prevent blocking the event loop.

2Use asynchronous operations and call next() only after completion.

3Keep middleware logic simple and fast to improve server response time.

Performance Quiz - 3 Questions

What is the main performance risk of using heavy synchronous code in custom middleware?

A It increases the size of the JavaScript bundle sent to the client.

B It blocks the event loop, delaying all incoming requests.

C It causes layout shifts in the browser.

D It improves server throughput by batching requests.

DevTools: Network and Performance panels

How to check: Use Network panel to measure server response time; use Performance panel to check request blocking time.

What to look for: Look for long server response times or blocking periods indicating slow middleware processing.

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