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.