Events vs callbacks decision in Node.js - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an event emitter instance.

Node.js

const EventEmitter = require('events');
const emitter = new [1]();

Drag options to blanks, or click blank then click option'

AEmitterEvent

BEmitter

CEvent

DEventEmitter

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong class name like 'Emitter' or 'Event'.

Forgetting to use parentheses to create an instance.

The EventEmitter class is used to create event emitter instances in Node.js.

2fill in blank

medium

Complete the code to add a callback function for the 'data' event.

Node.js

emitter.on('data', [1] => {
  console.log('Data received:', data);
});

Drag options to blanks, or click blank then click option'

A()

Bfunction(data)

Cdata

Devent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'function(data)' without arrow syntax here.

Using empty parentheses which means no parameters.

The callback function receives the event data as a parameter. Using data as the parameter name is common and clear.

3fill in blank

hard

Fix the error in the callback usage by completing the code.

Node.js

function fetchData(callback) {
  setTimeout(() => {
    const result = 'Hello';
    callback([1]);
  }, 1000);
}

Drag options to blanks, or click blank then click option'

Aresult

Bcallback

CfetchData

DsetTimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the callback function itself instead of data.

Passing unrelated function names.

The callback function should be called with the data result as argument.

4fill in blank

hard

Fill both blanks to emit an event with a message and listen for it.

Node.js

emitter.[1]('message', 'Hi there!');
emitter.[2]('message', msg => {
  console.log(msg);
});

Drag options to blanks, or click blank then click option'

Aemit

Bon

Csend

Dlisten

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'send' or 'listen' which are not Node.js EventEmitter methods.

Mixing up 'emit' and 'on' methods.

Use emit to send an event and on to listen for it.

5fill in blank

hard

Fill all three blanks to decide between callback and event for async handling.

Node.js

function processData(data, [1]) {
  if (data.length > 5) {
    [2]('error', new Error('Too long'));
  } else {
    [3](null, data);
  }
}

Drag options to blanks, or click blank then click option'

Acallback

Bemitter.emit

Demitter.on

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'emitter.on' instead of 'emitter.emit' to send events.

Confusing callback with event emitter methods.

Use a callback function to handle success or error. Use emitter.emit to send an error event.

Practice

1. Which statement best describes when to use callbacks versus events in Node.js?

easy

A. Use callbacks for simple single responses and events for multiple listeners.

B. Use events only for synchronous code and callbacks for asynchronous code.

C. Callbacks are for error handling only, events are for all other tasks.

D. Events replace callbacks completely in modern Node.js.

Solution

1. Step 1: Understand callbacks and events roles

   Callbacks run one function after a task finishes, suitable for simple, single responses.

2. Step 2: Understand events usage

   Events allow many listeners to respond to named signals, useful for complex or multiple reactions.

3. Final Answer:

   Use callbacks for simple single responses and events for multiple listeners. -> Option A

4. Quick Check:

   Callbacks = single response, Events = multiple listeners [OK]

Hint: Callbacks = one response; events = many listeners [OK]

Common Mistakes:

• Thinking events are only for synchronous code
• Believing callbacks handle all errors exclusively
• Assuming events completely replace callbacks

2. Which of the following is the correct syntax to add an event listener in Node.js?

easy

A. emitter.listen('eventName', callbackFunction);

B. emitter.addListener('eventName' callbackFunction);

C. emitter.callback('eventName', callbackFunction);

D. emitter.on('eventName', callbackFunction);

Solution

1. Step 1: Recall Node.js event listener syntax

   The standard method to add an event listener is using emitter.on with event name and callback.

2. Step 2: Check each option for syntax correctness

   emitter.on('eventName', callbackFunction); uses correct syntax with parentheses and comma. emitter.addListener('eventName' callbackFunction); misses a comma. Options A and D use invalid method names.

3. Final Answer:

   emitter.on('eventName', callbackFunction); -> Option D

4. Quick Check:

   Correct event listener syntax = emitter.on(...) [OK]

Hint: Use emitter.on('event', callback) for events [OK]

Common Mistakes:

• Missing comma between arguments
• Using incorrect method names like listen or callback
• Confusing addListener syntax without comma

3. What will be the output of this Node.js code snippet?

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('greet', () => console.log('Hello!'));
emitter.emit('greet');
emitter.emit('greet');

medium

A. Hello! printed once

B. No output

C. Hello! printed twice

D. Error thrown

Solution

1. Step 1: Understand event listener and emit behavior

   Each call to emitter.emit triggers all listeners for that event. Here, 'greet' event has one listener printing 'Hello!'.

2. Step 2: Count how many times emit is called

   emit('greet') is called twice, so the listener runs twice, printing 'Hello!' two times.

3. Final Answer:

   Hello! printed twice -> Option C

4. Quick Check:

   emit triggers listeners each time = output twice [OK]

Hint: emit calls listeners every time it's invoked [OK]

Common Mistakes:

• Thinking emit triggers listener only once
• Expecting no output without callback arguments
• Confusing event registration with callback invocation

4. Identify the error in this Node.js code using callbacks and events:

const EventEmitter = require('events');
const emitter = new EventEmitter();
function task(callback) {
  emitter.emit('done');
  callback();
}
task(() => console.log('Callback finished'));
emitter.on('done', () => console.log('Event done'));

medium

A. The event listener is added after task is called, so event may be missed.

B. Callback should be called before emitting the event.

C. setTimeout is used incorrectly without delay argument.

D. EventEmitter cannot be used with callbacks.

Solution

1. Step 1: Check order of event listener and task call

   The event listener is added after task() is called, so the 'done' event may emit before listener exists.

2. Step 2: Understand event emission timing

   task emits 'done' synchronously when called, but listener is added after task() call, so emit happens before listener setup.

3. Final Answer:

   The event listener is added after task is called, so event may be missed. -> Option A

4. Quick Check:

   Listener must be added before event emit [OK]

Hint: Add event listeners before emitting events [OK]

Common Mistakes:

• Adding listeners after emitting events
• Confusing callback order with event order
• Assuming setTimeout needs no delay argument

5. You want to notify multiple parts of your Node.js app when a file download finishes, but also run a cleanup callback once. Which approach fits best?

hard

A. Use only events for everything including cleanup.

B. Use an event emitter to notify multiple listeners and a callback for cleanup after download.

C. Use multiple callbacks for each notification and cleanup.

D. Use only a callback for all notifications and cleanup.

Solution

1. Step 1: Analyze notification needs

   Multiple parts need to be notified, which fits event emitters allowing many listeners.

2. Step 2: Analyze cleanup requirement

   Cleanup runs once after download, suitable for a single callback after task completion.

3. Step 3: Combine approaches

   Use events for multiple notifications and a callback for single cleanup to keep code clear and efficient.

4. Final Answer:

   Use an event emitter to notify multiple listeners and a callback for cleanup after download. -> Option B

5. Quick Check:

   Events = multiple notifications, callback = single cleanup [OK]

Hint: Events for many, callback for one-time cleanup [OK]

Common Mistakes:

• Trying to use only callbacks for multiple notifications
• Using multiple callbacks instead of events
• Using events for single cleanup unnecessarily