Handling uncaught exceptions in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch uncaught exceptions in Node.js.

Node.js

process.on('uncaughtException', (err) => {
  console.[1]('Caught exception:', err);
});

Drag options to blanks, or click blank then click option'

Ainfo

Berror

Cwarn

Dlog

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.log instead of console.error

Misspelling 'uncaughtException'

Not passing the error parameter

The process.on('uncaughtException') event handler uses console.error to log errors clearly.

2fill in blank

medium

Complete the code to exit the process after logging an uncaught exception.

Node.js

process.on('uncaughtException', (err) => {
  console.error(err);
  process.[1](1);
});

Drag options to blanks, or click blank then click option'

Aexit

Bterminate

Cend

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Using process.stop or process.end which do not exist

Not exiting the process after an uncaught exception

Use process.exit(1) to stop the Node.js process with an error code after handling the exception.

3fill in blank

hard

Fix the error in the code to properly handle uncaught exceptions.

Node.js

process.on('uncaughtException', (error) => {
  console.error('Error:', [1]);
});

Drag options to blanks, or click blank then click option'

Aerr

Berror.message

Cerror

Dexception

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than the parameter

Logging only error.message which hides stack trace

The callback parameter is named error, so use error to log the full error object.

4fill in blank

hard

Fill both blanks to log the error stack and then exit the process.

Node.js

process.on('uncaughtException', (err) => {
  console.[1](err.[2]);
  process.exit(1);
});

Drag options to blanks, or click blank then click option'

Aerror

Bstack

Cmessage

Dlog

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.log instead of console.error

Logging err.message instead of err.stack

Use console.error to log the error stack trace (err.stack) for detailed debugging.

5fill in blank

hard

Fill all three blanks to create a handler that logs the error, cleans up, and exits.

Node.js

process.on('uncaughtException', (err) => {
  console.[1]('Uncaught:', err.[2]);
  cleanup();
  process.[3](1);
});

Drag options to blanks, or click blank then click option'

Aerror

Bmessage

Cexit

Dlog

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.error but wanting a simple log message

Logging err.stack instead of err.message here

Forgetting to exit the process

Use console.log to print a simple message with err.message, then call process.exit(1) after cleanup.

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