Response time tracking middleware in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Express middleware log?

Consider this Express middleware that tracks response time. What will it log to the console when a request is made?

Express

function responseTimeLogger(req, res, next) {
  const start = Date.now();
  res.on('finish', () => {
    const duration = Date.now() - start;
    console.log(`Request took ${duration}ms`);
  });
  next();
}

A Logs the time in milliseconds before the request starts processing

B Logs the time in milliseconds it took to handle the request after the response finishes

C Logs the time in seconds it took to handle the request immediately when middleware runs

D Logs the time in milliseconds it took to handle the request before calling next()

💡 Hint

Think about when the 'finish' event on the response object happens.

The middleware records the start time, then waits for the 'finish' event on the response, which means the response has been sent. It then calculates the difference to find how long the request took.

📝 Syntax

intermediate

Identify the syntax error in this response time middleware

Which option correctly fixes the syntax error in this Express middleware code?

Express

function responseTime(req, res, next) {
  const start = Date.now()
  res.on('finish', () => {
    const duration = Date.now() - start
    console.log('Response time: ' + duration + 'ms')
  });
  next()
}

A Change the arrow function to a regular function to fix the syntax error

B Replace res.on with res.once to fix the syntax error

C Add a semicolon after next() to fix the syntax error

D Add a closing parenthesis after the res.on callback function before calling next()

💡 Hint

Check the parentheses balance around the res.on callback.

The res.on callback function is missing a closing parenthesis before next() is called, causing a syntax error.

🔧 Debug

advanced

Why does this middleware log zero for response time?

This middleware always logs 'Request took 0ms' even for slow requests. Why?

Express

function responseTime(req, res, next) {
  const start = Date.now();
  next();
  const duration = Date.now() - start;
  console.log(`Request took ${duration}ms`);
}

A Because the duration is calculated immediately after next(), before response finishes

B Because Date.now() returns time in seconds, not milliseconds

C Because next() blocks the event loop until response finishes

D Because the middleware does not listen to the 'close' event on res

💡 Hint

Think about when next() returns and when the response actually finishes.

Calling next() passes control to the next middleware but does not wait for the response to finish. The duration is measured immediately after next(), so it is near zero.

🧠 Conceptual

advanced

What is the best event to track response completion in Express middleware?

Which event on the response object should you listen to in order to accurately measure when the response has fully finished sending?

A 'close' event

B 'end' event

C 'finish' event

D 'data' event

💡 Hint

This event signals that all response data has been flushed.

The 'finish' event is emitted when the response has been sent completely. 'end' and 'data' are for readable streams, and 'close' can happen on errors or aborts.

❓ state_output

expert

What is the value of res.locals.duration after this middleware runs?

Given this middleware that tracks response time and stores it in res.locals, what will be the value of res.locals.duration after the response finishes?

Express

function responseTimeMiddleware(req, res, next) {
  const start = Date.now();
  res.on('finish', () => {
    res.locals.duration = Date.now() - start;
  });
  next();
}

// Assume this middleware is used in an Express app and a request takes 150ms to complete.

A Approximately 150 (milliseconds)

B Undefined, because res.locals is not set synchronously

C 0, because duration is calculated before response finishes

D NaN, because Date.now() returns a string

💡 Hint

res.locals can be set asynchronously inside event handlers.

The duration is calculated inside the 'finish' event handler, which runs after the response completes. So res.locals.duration will hold the actual time in milliseconds, about 150.