Error events and handling in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an 'error' event is emitted without a listener?

Consider this Node.js code snippet that emits an 'error' event on an EventEmitter without any listener attached. What will happen when this code runs?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.emit('error', new Error('Something went wrong'));

A The error event is queued until a listener is added.

B The error event is ignored silently.

C The error is logged to the console but the program continues.

D The program throws an uncaught error and crashes.

💡 Hint

Think about Node.js default behavior when an 'error' event has no listeners.

In Node.js, if an 'error' event is emitted and there is no listener for it, the process throws the error and crashes. This is to avoid silent failures.

❓ state_output

intermediate

What is the output after handling an error event?

Given this code, what will be printed to the console?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('error', (err) => {
  console.log('Caught error:', err.message);
});
emitter.emit('error', new Error('Oops!'));

A Uncaught Error: Oops!

B Caught error: Oops!

C Nothing is printed.

D Caught error: undefined

💡 Hint

Check how the error message is accessed inside the listener.

The listener catches the error event and logs the message property of the error object, which is 'Oops!'.

📝 Syntax

advanced

Which option correctly attaches an error event listener?

Select the code snippet that correctly attaches an 'error' event listener to an EventEmitter instance named 'emitter'.

A emitter.addListener('error', err => console.log(err.message));

B emitter.on('error', function(err) { console.log(err); });

C emitter.error(function(err) { console.log(err); });

D emitter.listen('error', (err) => console.log(err));

💡 Hint

Check the correct method names for attaching event listeners in Node.js EventEmitter.

Both on and addListener are valid methods to attach listeners. Option A uses addListener correctly with an arrow function. Option B is also valid syntax. Options C and D use invalid method names.

🔧 Debug

advanced

Why does this error handler not catch the error?

Look at this code snippet. Why does the error handler not catch the emitted error?

Node.js

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.emit('error', new Error('Fail'));
emitter.on('error', (err) => {
  console.log('Error caught:', err.message);
});

A The error event was emitted before the listener was attached.

B The listener function has a syntax error.

C The error object is not passed correctly to the listener.

D The EventEmitter instance does not support error events.

💡 Hint

Consider the order of event emission and listener attachment.

Events in Node.js are synchronous. If an event is emitted before a listener is attached, that listener will not catch the event.

🧠 Conceptual

expert

What is the best practice to avoid crashing on error events in Node.js?

Which approach best prevents the Node.js process from crashing due to unhandled 'error' events on EventEmitters?

A Use process.on('uncaughtException') to handle all errors globally.

B Wrap every emit call in try-catch blocks.

C Always attach an 'error' event listener to every EventEmitter instance.

D Ignore error events and let the process crash to fix bugs quickly.

💡 Hint

Think about how Node.js expects error events to be handled on EventEmitters.

Node.js expects every EventEmitter that can emit 'error' events to have an 'error' listener attached. This prevents the process from crashing due to unhandled errors. Using try-catch around emit calls is ineffective because emit is synchronous but listeners run asynchronously. Global handlers like 'uncaughtException' are last-resort and not recommended for normal error handling.