Unhandled rejection handling in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an unhandled rejection in Node.js?

An unhandled rejection happens when a Promise is rejected but no code catches or handles that rejection. It can cause the program to crash or behave unexpectedly.

beginner

How do you listen for unhandled promise rejections globally in Node.js?

You use the 'unhandledRejection' event on the process object, like this: process.on('unhandledRejection', (reason, promise) => { /* handle it */ });

beginner

Why is it important to handle unhandled rejections?

Because unhandled rejections can cause your app to crash or leave errors unnoticed, handling them helps keep your app stable and lets you log or fix errors properly.

intermediate

What is a common way to handle unhandled rejections in production?

A common way is to log the error details and then gracefully shut down the app or restart it to avoid running in a bad state.

beginner

Show a simple example of handling unhandled rejections in Node.js.

process.on('unhandledRejection', (reason, promise) => { console.error('Unhandled Rejection at:', promise, 'reason:', reason); // Optional: process.exit(1) to stop the app });

What event does Node.js emit when a promise rejection is not handled?

A unhandledRejection

B error

C rejectionHandled

D promiseRejected

Node.js emits the 'unhandledRejection' event on the process object when a promise rejection is not handled.

What happens if you do NOT handle unhandled promise rejections in Node.js?

A Node.js automatically retries the promise

B Nothing, the rejection is ignored safely

C The app may crash or behave unpredictably

D The promise resolves successfully

Unhandled rejections can cause the app to crash or behave unpredictably if not handled.

Which object do you use to listen for unhandled promise rejections?

A global

B Promise

C console

D process

The 'process' object emits the 'unhandledRejection' event.

What parameters does the 'unhandledRejection' event handler receive?

A error, stack

B reason, promise

C event, data

D message, code

The handler receives the rejection reason (error) and the promise that was rejected.

What is a good practice after catching an unhandled rejection in production?

A Log the error and gracefully shut down or restart the app

B Ignore it and continue running

C Throw a new error immediately

D Automatically resolve the promise

Logging and graceful shutdown help maintain app stability after errors.

Explain what an unhandled rejection is and why it matters in Node.js applications.

Think about what happens when a promise fails but no one catches the error.

Describe how to set up a global handler for unhandled promise rejections in Node.js and what you might do inside that handler.

Consider the event name and what actions help keep the app safe.

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