Unhandled rejection handling in Node.js - Performance & Optimization

Performance: Unhandled rejection handling

HIGH IMPACT

This affects the responsiveness and stability of Node.js applications by preventing crashes and unresponsive states caused by unhandled promise rejections.

Handling promise rejections in asynchronous code

Node.js

async function fetchData() {
  try {
    const data = await fetch('https://api.example.com/data');
    return await data.json();
  } catch (error) {
    console.error('Fetch failed:', error);
    return null;
  }
}

fetchData();

Catches errors preventing unhandled rejections and keeps event loop responsive.

📈 Performance GainPrevents event loop blocking and process crashes

Handling promise rejections in asynchronous code

Node.js

async function fetchData() {
  const data = await fetch('https://api.example.com/data');
  return data.json();
}

fetchData(); // No catch or error handling

Unhandled promise rejection can crash the Node.js process or cause unresponsive behavior.

📉 Performance CostBlocks event loop on error, causing delayed response or crash

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No rejection handling	N/A	N/A	N/A	[X] Bad
Local try/catch with async/await	N/A	N/A	N/A	[OK] Good
Global unhandledRejection logging	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

Unhandled promise rejections affect the Node.js event loop by potentially blocking it or causing crashes, which delays processing of other events and degrades responsiveness.

→Event Loop

→Error Handling

⚠️ BottleneckEvent Loop blocking due to unhandled errors

Core Web Vital Affected

INP

This affects the responsiveness and stability of Node.js applications by preventing crashes and unresponsive states caused by unhandled promise rejections.

Optimization Tips

1Always handle promise rejections locally with try/catch or .catch()

2Use process.on('unhandledRejection') to log and monitor missed errors

3Never ignore unhandled rejections to avoid event loop blocking or crashes

Performance Quiz - 3 Questions

What is the main performance risk of not handling promise rejections in Node.js?

A The browser will repaint more often

B The event loop can block or crash the process

C CSS selectors become slower

D The network requests increase in size

DevTools: Node.js Inspector (Debugger) and Console

How to check: Run your Node.js app with --inspect flag, open DevTools, and monitor console for unhandled rejection warnings or errors.

What to look for: Look for 'UnhandledPromiseRejectionWarning' or logged errors indicating unhandled rejections.

Practice

1. What is the main purpose of using process.on('unhandledRejection') in a Node.js application?

easy

A. To catch errors from promises that were not handled anywhere else

B. To handle synchronous errors in try-catch blocks

C. To restart the Node.js server automatically

D. To log all successful promise resolutions

Solution

1. Step 1: Understand what unhandled rejections are

   Unhandled rejections happen when a promise fails but no .catch() or try-catch handles the error.

2. Step 2: Role of process.on('unhandledRejection')

   This event listener catches those unhandled promise errors so you can log or clean up before the app crashes.

3. Final Answer:

   To catch errors from promises that were not handled anywhere else -> Option A

4. Quick Check:

   Unhandled promise errors = process.on('unhandledRejection') [OK]

Hint: Unhandled promise errors are caught by process.on('unhandledRejection') [OK]

Common Mistakes:

• Confusing unhandledRejection with synchronous try-catch
• Thinking it restarts the server automatically
• Assuming it logs successful promises

2. Which of the following is the correct syntax to listen for unhandled promise rejections in Node.js?

easy

A. process.on('unhandledRejection', handlerFunction)

B. process.catch('unhandledRejection', handlerFunction)

C. process.listen('unhandledRejection', handlerFunction)

D. process.handle('unhandledRejection', handlerFunction)

Solution

1. Step 1: Recall Node.js event listening syntax

   Node.js uses process.on(eventName, callback) to listen to events.

2. Step 2: Match the event name and method

   The event for unhandled promise rejections is 'unhandledRejection', so the correct syntax is process.on('unhandledRejection', handlerFunction).

3. Final Answer:

   process.on('unhandledRejection', handlerFunction) -> Option A

4. Quick Check:

   Event listening in Node.js = process.on() [OK]

Hint: Use process.on('unhandledRejection', handler) to catch promise errors [OK]

Common Mistakes:

• Using process.catch instead of process.on
• Using process.listen or process.handle which don't exist
• Mixing event name spelling

3. Consider this Node.js code snippet:

process.on('unhandledRejection', (reason) => {
  console.log('Error:', reason.message);
});

Promise.reject(new Error('Failed promise'));

What will be printed to the console?

medium

A. No output, program crashes silently

B. Error: Failed promise

C. Error: undefined

D. Unhandled promise rejection

Solution

1. Step 1: Understand the unhandledRejection event handler

   The handler logs the error message from the rejection reason.

2. Step 2: Analyze the rejected promise

   The promise rejects with new Error('Failed promise'), so reason.message is 'Failed promise'.

3. Final Answer:

   Error: Failed promise -> Option B

4. Quick Check:

   Rejected error message logged = 'Error: Failed promise' [OK]

Hint: Unhandled rejection logs error.message from rejected Error object [OK]

Common Mistakes:

• Expecting no output or silent crash
• Confusing reason.message with undefined
• Thinking the event logs generic text

4. You wrote this code to catch unhandled promise rejections:

process.on('unhandledRejection', (error) => {
  console.log('Caught:', error);
});

Promise.reject('Oops!');

But the console shows Caught: Oops! instead of an error message. What is the issue?

medium

A. The handler function must be async to catch rejections

B. The event name should be 'unhandledRejections' (plural)

C. You must use try-catch instead of process.on for promises

D. The rejection reason is a string, not an Error object, so error.message is undefined

Solution

1. Step 1: Check the rejection reason type

   The promise rejects with a string 'Oops!', not an Error object.

2. Step 2: Understand how the handler logs the error

   The handler logs the whole error variable, which is the string 'Oops!', so it prints exactly that.

3. Final Answer:

   The rejection reason is a string, not an Error object, so error.message is undefined -> Option D

4. Quick Check:

   Rejection reason type affects error.message presence [OK]

Hint: Rejection reason can be any type; strings have no .message [OK]

Common Mistakes:

• Using wrong event name 'unhandledRejections'
• Thinking async handler is required
• Believing try-catch catches unhandled rejections

5. You want to log unhandled promise rejections and then gracefully shut down your Node.js server. Which code snippet correctly implements this behavior?

hard

A. process.on('unhandledRejection', (reason) => {
console.error('Unhandled rejection:', reason);
process.exit(1);
});

B. process.on('unhandledRejection', (reason) => {
console.log('Handled rejection:', reason.message);
});

C. process.on('unhandledRejection', (reason) => {
console.error('Unhandled rejection:', reason);
server.close(() => process.exit(1));
});

D. process.on('unhandledRejection', async (reason) => {
await server.close();
console.log('Server closed');
});

Solution

1. Step 1: Understand graceful shutdown

   Graceful shutdown means closing server connections before exiting the process.

2. Step 2: Analyze each option's shutdown approach

   process.on('unhandledRejection', (reason) => { console.error('Unhandled rejection:', reason); server.close(() => process.exit(1)); }); logs the error, then calls server.close() with a callback to exit after closing. This is correct.

3. Step 3: Identify why others are incorrect

   process.on('unhandledRejection', (reason) => { console.error('Unhandled rejection:', reason); process.exit(1); }); exits immediately without closing server; C only logs without shutdown; D awaits server.close but does not exit process.

4. Final Answer:

   process.on('unhandledRejection', (reason) => { console.error('Unhandled rejection:', reason); server.close(() => process.exit(1)); }); -> Option C

5. Quick Check:

   Graceful shutdown = server.close() then exit [OK]

Hint: Close server before exit to shutdown gracefully [OK]

Common Mistakes:

• Exiting process immediately without closing server
• Not calling process.exit after closing server
• Logging without shutting down server