Handling uncaught exceptions in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an uncaught exception in Node.js?

An uncaught exception is an error that occurs during program execution but is not handled by any try-catch block or error handler, causing the program to crash if not managed.

beginner

How do you listen for uncaught exceptions in Node.js?

You use the process object's 'uncaughtException' event to listen for errors that were not caught anywhere else in the code.

intermediate

Why should you avoid using 'uncaughtException' for normal error handling?

Because it is a last-resort handler; the application state might be unstable after an uncaught exception, so it's better to log the error and safely shut down the app.

beginner

What is the recommended way to handle errors in asynchronous code in Node.js?

Use try-catch blocks with async/await or handle errors in callbacks/promises instead of relying on 'uncaughtException'.

beginner

Show a simple example of handling an uncaught exception in Node.js.

process.on('uncaughtException', (err) => { console.error('Caught exception:', err); process.exit(1); // Exit to avoid unstable state });

Which Node.js event is used to catch uncaught exceptions?

A 'unhandledRejection'

B 'error'

C 'exceptionCaught'

D 'uncaughtException'

The 'uncaughtException' event on the process object catches exceptions not handled elsewhere.

What should you do after catching an uncaught exception?

A Log the error and safely shut down the application

B Ignore the error

C Log the error and continue running normally

D Restart the Node.js process automatically

After an uncaught exception, the app might be unstable, so it's best to log and exit safely.

Which of these is NOT a good practice for error handling in Node.js?

A Using try-catch in async functions

B Relying only on 'uncaughtException' for all errors

C Handling promise rejections properly

D Using callbacks with error parameters

'uncaughtException' should be a last resort, not the main error handling method.

What happens if an uncaught exception is not handled in Node.js?

A The program logs the error and continues

B The program continues silently

C The program crashes immediately

D The program restarts automatically

Without handling, uncaught exceptions cause the Node.js process to crash.

Which event is used to catch unhandled promise rejections in Node.js?

A 'unhandledRejection'

B 'rejectionHandled'

C 'error'

D 'uncaughtException'

'unhandledRejection' event handles promises rejected without catch.

Explain how to handle uncaught exceptions in Node.js and why it is important.

Think about what happens if an error is not caught and how Node.js lets you listen for it.

Describe best practices for error handling in asynchronous Node.js code compared to using uncaughtException.

Consider how errors flow in async code and how to catch them early.

Practice

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

easy

A. To handle HTTP requests automatically

B. To log user activity in the application

C. To restart the server after every request

D. To catch errors that were not handled anywhere else in the code

Solution

1. Step 1: Understand the role of uncaught exceptions

   Uncaught exceptions are errors that happen but are not caught by any try-catch block or error handler in the code.

2. Step 2: Purpose of process.on('uncaughtException')

   This event listener catches those uncaught errors to prevent the app from crashing unexpectedly.

3. Final Answer:

   To catch errors that were not handled anywhere else in the code -> Option D

4. Quick Check:

   Uncaught exceptions = catch unhandled errors [OK]

Hint: Remember: uncaughtException catches errors missed by try-catch [OK]

Common Mistakes:

• Thinking it handles normal HTTP requests
• Assuming it restarts the server automatically
• Confusing it with logging user actions

2. Which of the following is the correct syntax to listen for uncaught exceptions in Node.js?

easy

A. process.listen('uncaughtException', (err) => { console.error(err); });

B. process.catch('uncaughtException', (err) => { console.error(err); });

C. process.on('uncaughtException', (err) => { console.error(err); });

D. process.handle('uncaughtException', (err) => { console.error(err); });

Solution

1. Step 1: Identify the correct event listener method

   Node.js uses process.on to listen for events like 'uncaughtException'.

2. Step 2: Verify the event name and callback

   The event name is exactly 'uncaughtException' and the callback receives the error object.

3. Final Answer:

   process.on('uncaughtException', (err) => { console.error(err); }); -> Option C

4. Quick Check:

   Use process.on for events [OK]

Hint: Use process.on for event listening, not catch or listen [OK]

Common Mistakes:

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

3. Consider the following Node.js code snippet:

process.on('uncaughtException', (err) => {
  console.log('Caught:', err.message);
});

throw new Error('Oops!');

What will be the output when this code runs?

medium

A. The program crashes without any output

B. Caught: Oops!

C. Error: Oops!

D. No output, the error is ignored

Solution

1. Step 1: Understand the uncaughtException handler

   The handler logs the error message prefixed with 'Caught:'.

2. Step 2: The thrown error triggers the handler

   The thrown error 'Oops!' is caught by the listener and logged as 'Caught: Oops!'.

3. Final Answer:

   Caught: Oops! -> Option B

4. Quick Check:

   Uncaught error triggers handler output [OK]

Hint: Thrown error triggers uncaughtException handler output [OK]

Common Mistakes:

• Expecting the program to crash immediately
• Thinking the error message prints with 'Error:' prefix
• Assuming no output because error is ignored

4. You wrote this code to catch uncaught exceptions:

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

setTimeout(() => {
  throw new Error('Fail');
}, 1000);

But the program crashes after the error is logged. What is the best fix?

medium

A. Add process.exit(1); inside the handler after logging

B. Remove the throw statement inside setTimeout

C. Wrap the throw inside a try-catch block

D. Use process.on('error') instead of uncaughtException

Solution

1. Step 1: Understand uncaughtException behavior

   After catching, the app is in an unstable state and should exit safely.

2. Step 2: Add process.exit(1) to stop the app

   Calling process.exit(1) after logging ensures the app stops cleanly.

3. Final Answer:

   Add process.exit(1); inside the handler after logging -> Option A

4. Quick Check:

   Exit after uncaughtException to avoid unstable state [OK]

Hint: Exit process after uncaughtException to avoid crashes [OK]

Common Mistakes:

• Ignoring the need to exit after catching
• Trying to catch error inside setTimeout instead
• Using wrong event name 'error' instead of 'uncaughtException'

5. You want to log uncaught exceptions and then restart your Node.js server automatically. Which approach correctly combines handling uncaught exceptions and restarting the app?

hard

A. Use process.on('uncaughtException') to log error, then call process.exit(1), and use a separate script or tool to restart the server

B. Inside uncaughtException handler, just call server.listen() again to restart

C. Wrap the entire app code in a try-catch block to restart on error

D. Use process.on('exit') to catch errors and restart the server

Solution

1. Step 1: Handle uncaught exceptions by logging and exiting

   Inside the handler, log the error and call process.exit(1) to stop the app safely.

2. Step 2: Use an external tool or script to restart the server

   Node.js itself does not restart automatically; tools like PM2 or systemd can restart on exit.

3. Final Answer:

   Use process.on('uncaughtException') to log error, then call process.exit(1), and use a separate script or tool to restart the server -> Option A

4. Quick Check:

   Log + exit + external restart tool = correct approach [OK]

Hint: Exit on error, restart externally (PM2/systemd) for stability [OK]

Common Mistakes:

• Trying to restart server inside uncaughtException handler
• Using process.on('exit') to catch errors (wrong event)
• Wrapping entire app in try-catch which misses async errors