Unhandled rejection handling in Node.js - Mini Project: Build & Apply

Handling Unhandled Promise Rejections in Node.js

📖 Scenario: You are building a simple Node.js script that uses promises to simulate asynchronous tasks. Sometimes, promises might reject without proper handling, causing unhandled rejection warnings or crashes.To make your application more reliable, you want to catch and handle any unhandled promise rejections globally.

🎯 Goal: Create a Node.js script that defines a promise which rejects without a catch, then add a global handler for unhandled promise rejections to log a friendly message.

📋 What You'll Learn

Create a promise that rejects with the message 'Task failed!'

Add a global event listener for 'unhandledRejection' on the process object

Log the rejection reason with a clear message inside the event listener

Trigger the unhandled rejection by not catching the promise rejection

💡 Why This Matters

🌍 Real World

In real Node.js applications, unhandled promise rejections can cause crashes or silent failures. Handling them globally helps keep the app stable and easier to debug.

💼 Career

Understanding unhandled rejection handling is important for backend developers to write robust asynchronous code and maintain production server stability.

1

Create a promise that rejects

Create a constant called failingPromise that is a new Promise which immediately rejects with the string 'Task failed!'.

Node.js

// Create a constant called failingPromise that rejects immediately
// Your code here

Hint

Use new Promise((resolve, reject) => { reject('Task failed!'); }) to create the promise.

2

Add a global unhandledRejection event listener

Add a listener on the process object for the 'unhandledRejection' event. The listener should be a function with parameters reason and promise.

Node.js

const failingPromise = new Promise((resolve, reject) => {
  reject('Task failed!');
});

// Add a process event listener for 'unhandledRejection' here
// Your code here

Hint

Use process.on('unhandledRejection', (reason, promise) => { ... }) to listen globally.

3

Log the rejection reason inside the event listener

Inside the unhandledRejection event listener, add a console.log statement that outputs the string 'Unhandled rejection:' followed by the reason parameter.

Node.js

const failingPromise = new Promise((resolve, reject) => {
  reject('Task failed!');
});

process.on('unhandledRejection', (reason, promise) => {
  // Log the rejection reason here
  // Your code here
});

Hint

Use console.log('Unhandled rejection:', reason) inside the listener function.

4

Trigger the unhandled rejection by not catching the promise

Do not add any catch or then handlers to failingPromise. This will cause the rejection to be unhandled and trigger the global listener.

Node.js

const failingPromise = new Promise((resolve, reject) => {
  reject('Task failed!');
});

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled rejection:', reason);
});

// Do not add catch or then to failingPromise
// Your code here

Hint

Simply leave failingPromise without any catch or then handlers.

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