Unhandled rejection handling in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to listen for unhandled promise rejections.

Node.js

process.on('unhandledRejection', [1] => {
  console.error('Unhandled rejection:', [1]);
});

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

Ae

Berr

Creason

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name that is not descriptive like 'e' or 'err' can confuse the purpose.

Not handling the event at all causes the process to crash on unhandled rejections.

The event listener for 'unhandledRejection' receives the reason why the promise was rejected.

2fill in blank

medium

Complete the code to exit the process after logging an unhandled rejection.

Node.js

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

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

Aexit

Bquit

Cstop

Dend

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like 'stop' or 'quit' causes runtime errors.

Not exiting the process may leave the app in an unstable state.

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

3fill in blank

hard

Fix the error in the unhandled rejection handler to correctly log the error stack.

Node.js

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

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

Areason.toString()

Breason.error

Creason.message

Dreason.stack

Attempts:

3 left

💡 Hint

Common Mistakes

Logging 'reason.message' instead of 'reason.stack' loses debugging details.

Accessing a non-existent property like 'reason.error' causes undefined output.

The stack property of the error object shows the full stack trace for debugging.

4fill in blank

hard

Fill both blanks to create a handler that logs the error and then exits the process.

Node.js

process.on('unhandledRejection', ([1]) => {
  console.error('Unhandled rejection:', [2]);
  process.exit(1);
});

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

Areason

Berror

Cerr

De

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and the logged variable causes reference errors.

Using generic names like 'e' can reduce code readability.

Using the same parameter name reason consistently helps clarity and correctness.

5fill in blank

hard

Fill all three blanks to create a robust unhandled rejection handler that logs the error stack, warns, and exits.

Node.js

process.on('unhandledRejection', ([1]) => {
  console.error('Unhandled rejection stack:', [2]);
  console.warn('Warning: process will exit due to unhandled rejection');
  process.[3](1);
});

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

Areason

Breason.stack

Cexit

Derr

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'err' instead of 'reason' can confuse the event parameter.

Logging 'reason' instead of 'reason.stack' loses stack trace info.

Calling a non-existent method instead of 'exit' causes runtime errors.

The handler uses reason as the parameter, logs reason.stack for details, and calls process.exit(1) to stop the app.

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