Response time optimization in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Event Loop Impact on Response Time

In Node.js, which scenario best explains why a long-running synchronous function can increase response time for incoming requests?

A The event loop prioritizes new requests over synchronous functions, so response time is unaffected.

B The function runs in a separate thread, so it does not affect response time.

C Node.js automatically splits synchronous functions into smaller tasks to avoid blocking.

D The event loop is blocked, so new requests must wait until the function finishes.

💡 Hint

Think about how Node.js handles tasks in a single thread.

Node.js uses a single-threaded event loop. If a synchronous function runs for a long time, it blocks the event loop. This means new incoming requests cannot be processed until the function finishes, increasing response time.

❓ component_behavior

intermediate

Effect of Async/Await on Response Time

Consider this Node.js code snippet handling HTTP requests. What is the main benefit of using async/await here for response time optimization?

Node.js

const http = require('http');

http.createServer(async (req, res) => {
  const data = await fetchDataFromDB();
  res.end(data);
}).listen(3000);

A It allows the server to handle other requests while waiting for the database response.

B It makes the database query run faster.

C It blocks the event loop until the database returns data.

D It caches the database response to reduce future response times.

💡 Hint

Think about what async/await does to the event loop.

Using async/await lets Node.js pause the current request handler without blocking the event loop. This allows the server to process other incoming requests while waiting for the database, improving overall response time.

🔧 Debug

advanced

Identifying Cause of Slow Response in Middleware

Given this Express middleware, what is the cause of increased response time?

Node.js

app.use((req, res, next) => {
  const start = Date.now();
  while (Date.now() - start < 100) {
    // busy wait for 100ms
  }
  next();
});

A The middleware calls next() too early, causing race conditions.

B The middleware uses asynchronous code incorrectly, causing delays.

C The busy wait blocks the event loop, delaying all requests.

D The middleware does not call next(), so requests hang indefinitely.

💡 Hint

Consider what happens when the event loop is busy waiting.

The busy wait loop blocks the event loop for 100ms on every request. This prevents Node.js from handling other requests during that time, increasing response time significantly.

📝 Syntax

advanced

Correct Use of setImmediate for Response Time

Which option correctly uses setImmediate to defer a CPU-heavy task without blocking the response?

Node.js

app.get('/heavy', (req, res) => {
  // CPU-heavy task here
  res.end('Done');
});

A

app.get('/heavy', (req, res) => {
  heavyTask();
  setImmediate(() => res.end('Done'));
});

B

app.get('/heavy', (req, res) => {
  setImmediate(() => {
    heavyTask();
  });
  res.end('Done');
});

C

app.get('/heavy', (req, res) => {
  setTimeout(() => {
    heavyTask();
    res.end('Done');
  }, 0);
});

D

app.get('/heavy', (req, res) => {
  res.end('Done');
  heavyTask();
});

💡 Hint

The response should be sent immediately, and the heavy task deferred.

Option B defers the CPU-heavy task using setImmediate, allowing the response to be sent immediately. This prevents blocking the event loop and improves response time.

❓ state_output

expert

Analyzing Response Time with Caching and Async Calls

Consider this Node.js code snippet using a simple cache. What will be the output time difference between the first and second request to /data endpoint?

Node.js

const http = require('http');
let cache = null;

async function fetchData() {
  return new Promise(resolve => setTimeout(() => resolve('data'), 100));
}

http.createServer(async (req, res) => {
  if (req.url === '/data') {
    if (cache) {
      res.end(cache);
    } else {
      const data = await fetchData();
      cache = data;
      res.end(data);
    }
  } else {
    res.end('Not found');
  }
}).listen(3000);

A First request takes about 100ms; second request is almost instant due to cache.

B Both requests take about 100ms because fetchData is called every time.

C First request is instant; second request takes 100ms because of cache delay.

D Both requests are instant because cache is prefilled.

💡 Hint

Think about when fetchData is called and when cache is used.

The first request calls fetchData, which takes 100ms. The result is stored in cache. The second request uses the cached data and responds immediately, reducing response time.