Unhandled rejection handling in Node.js - Step-by-Step Execution

Concept Flow - Unhandled rejection handling

Promise created

↓

Promise rejected?

No→Promise resolved, normal flow

Yes↓

Is rejection handled?

Yes→Handle rejection with .catch() or try/catch

No↓

Node.js emits 'unhandledRejection' event

↓

Optional process exit or logging

↓

Program continues or stops

This flow shows how Node.js handles promises that reject without a catch handler, emitting an event if unhandled.

Execution Sample

Node.js

const p = Promise.reject(new Error('fail'));
// No catch handler

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

This code creates a rejected promise without a catch, triggering the unhandledRejection event.

Execution Table

Step	Action	Promise State	Handler Present?	Event Emitted	Output
1	Create rejected promise	Rejected	No	No
2	No catch handler attached	Rejected	No	No
3	Event loop checks for unhandled rejections	Rejected	No	Yes	unhandledRejection event emitted
4	Event listener logs rejection reason	Rejected	No	Yes	Unhandled rejection: fail
5	Program continues or exits based on policy	Rejected	No	Yes

💡 Unhandled rejection detected with no handler, event emitted and logged.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
p	undefined	Rejected Promise(Error: fail)	Rejected Promise(Error: fail)	Rejected Promise(Error: fail)	Rejected Promise(Error: fail)
handlerAttached	false	false	false	false	false
eventEmitted	false	false	false	true	true

Key Moments - 3 Insights

Why does Node.js emit an 'unhandledRejection' event?

Because the promise was rejected but no catch handler was attached, as shown in execution_table step 3.

What happens if we add a .catch() handler after the rejection?

The rejection is handled, so the 'unhandledRejection' event is not emitted. This is because handlerAttached would be true, preventing event emission.

Does the program stop immediately after an unhandled rejection?

Not necessarily. Node.js emits the event and logs it, but the program continues unless explicitly exited, as shown in execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the 'unhandledRejection' event emitted?

A Step 1

B Step 3

C Step 5

D Step 2

💡 Hint

Check the 'Event Emitted' column in the execution_table rows.

According to variable_tracker, what is the value of 'handlerAttached' after step 2?

A true

B undefined

C false

D null

💡 Hint

Look at the 'handlerAttached' row and the 'After Step 2' column.

If a .catch() handler is added immediately after promise creation, how would the 'eventEmitted' variable change?

A It would become false

B It would remain true

C It would become undefined

D It would become null

💡 Hint

Refer to the explanation in key_moments about handler presence preventing event emission.

Concept Snapshot

Unhandled rejection handling in Node.js:
- When a Promise rejects without a catch, Node.js emits 'unhandledRejection'.
- Attach .catch() or use try/catch with async/await to handle rejections.
- Listen to 'unhandledRejection' on process to log or handle globally.
- Program continues unless explicitly exited.
- Helps avoid silent failures in async code.

Full Transcript

In Node.js, when a Promise is rejected but no catch handler is attached, the runtime emits an 'unhandledRejection' event. This event allows developers to detect and log errors that would otherwise be missed. The example code creates a rejected Promise without a catch handler, triggering this event. The execution table shows the promise state and event emission steps. Variables track the promise state, handler presence, and event emission. Key moments clarify why the event is emitted, what happens if a handler is added, and that the program does not stop immediately. The visual quiz tests understanding of when the event fires and variable states. This helps beginners see how unhandled rejections flow through Node.js and how to handle them properly.