Custom event emitter classes in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the EventEmitter class from the 'events' module.

Node.js

const [1] = require('events');

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

AEventEmitter

BEmitter

CEvent

DEmitterClass

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect class names like 'Emitter' or 'Event'.

Forgetting to require the 'events' module.

The EventEmitter class is imported from the 'events' module using const EventEmitter = require('events');.

2fill in blank

medium

Complete the code to create a new class named MyEmitter that extends EventEmitter.

Node.js

class MyEmitter extends [1] {
  constructor() {
    super();
  }
}

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

AEmitter

BEventEmitter

CEvent

DEmitterClass

Attempts:

3 left

💡 Hint

Common Mistakes

Extending a wrong or undefined class.

Forgetting to call super() in the constructor.

The custom class should extend EventEmitter to inherit event capabilities.

3fill in blank

hard

Fix the error in the code to emit a 'start' event from the emitter instance.

Node.js

const emitter = new MyEmitter();
emitter.[1]('start');

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

Aemit

Bfire

CemitEvent

Dtrigger

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like 'trigger' or 'fire'.

Using 'emitEvent' which is not a method.

The correct method to emit an event is emit.

4fill in blank

hard

Fill both blanks to add a listener for the 'data' event that logs the received message.

Node.js

emitter.[1]('data', ([2]) => {
  console.log([2]);
});

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

Aon

Bmessage

Cdata

Dlisten

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'addListener' instead of 'on' (valid but not in options).

Using the event name as the parameter name.

The on method adds a listener, and the parameter name can be any valid identifier like 'message'.

5fill in blank

hard

Fill all three blanks to create an instance, add a listener for 'end', and emit the 'end' event.

Node.js

const [1] = new MyEmitter();
[1].on('end', () => {
  console.log('Finished');
});
[1].[2]('end');

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

Aemitter

Bemit

Ctrigger

Dfire

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names like 'trigger' or 'fire'.

Using different variable names inconsistently.

Create an instance named 'emitter', use on to add a listener, and emit to trigger the event.

Practice

1. What is the main purpose of creating a custom event emitter class in Node.js?

easy

A. To replace the need for functions and callbacks

B. To allow different parts of your program to communicate by sending and listening to events

C. To speed up the execution of synchronous code

D. To automatically handle HTTP requests

Solution

1. Step 1: Understand event emitters

   Event emitters let parts of a program send signals (events) and others listen and react to them.

2. Step 2: Purpose of custom event emitter classes

   Custom classes extend EventEmitter to organize and manage these events clearly.

3. Final Answer:

   To allow different parts of your program to communicate by sending and listening to events -> Option B

4. Quick Check:

   Event communication = C [OK]

Hint: Event emitters enable communication between code parts [OK]

Common Mistakes:

• Thinking event emitters speed up synchronous code
• Confusing event emitters with HTTP handling
• Believing event emitters replace all functions

2. Which of the following is the correct way to listen for an event named data in a custom event emitter instance myEmitter?

easy

A. myEmitter.on('data', callback)

B. myEmitter.emit('data', callback)

C. myEmitter.listen('data', callback)

D. myEmitter.trigger('data', callback)

Solution

1. Step 1: Identify the method to listen for events

   The on method is used to register a callback for an event.

2. Step 2: Check other options

   emit triggers events, listen and trigger are not valid EventEmitter methods.

3. Final Answer:

   myEmitter.on('data', callback) -> Option A

4. Quick Check:

   Listen with on() = A [OK]

Hint: Use on() to listen, emit() to send events [OK]

Common Mistakes:

• Using emit() to listen instead of on()
• Using non-existent methods like listen() or trigger()
• Confusing event names with method names

3. Consider this code snippet:

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const emitter = new MyEmitter();
emitter.on('greet', name => console.log(`Hello, ${name}!`));
emitter.emit('greet', 'Alice');

What will be printed when this code runs?

medium

A. greet Alice

B. Error: greet event not found

C. Hello, Alice!

D. undefined

Solution

1. Step 1: Understand event registration

   The on method registers a listener for 'greet' that prints a greeting with the name.

2. Step 2: Understand event emission

   The emit method triggers 'greet' with argument 'Alice', so the listener runs and prints the message.

3. Final Answer:

   Hello, Alice! -> Option C

4. Quick Check:

   Emit triggers listener output = B [OK]

Hint: emit() runs on() listeners with given arguments [OK]

Common Mistakes:

• Expecting event name or arguments to print directly
• Confusing emit() with on()
• Assuming error if no listener exists

4. What is wrong with this custom event emitter code?

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const emitter = new MyEmitter();
emitter.emit('start');
emitter.on('start', () => console.log('Started'));

medium

A. The event name 'start' is invalid

B. emit() cannot be called without arguments

C. You must call super() in the constructor of MyEmitter

D. The event listener is registered after the event is emitted, so it won't run

Solution

1. Step 1: Check event listener registration timing

   The listener for 'start' is added after the event is emitted, so it misses the event.

2. Step 2: Validate other options

   emit() can be called without arguments, super() is optional if no constructor, and 'start' is a valid event name.

3. Final Answer:

   The event listener is registered after the event is emitted, so it won't run -> Option D

4. Quick Check:

   Listener after emit = no output = D [OK]

Hint: Register listeners before emitting events [OK]

Common Mistakes:

• Calling emit() before on() listener
• Thinking emit() needs arguments always
• Assuming constructor must call super() if none defined

5. You want to create a custom event emitter class that counts how many times an event named ping is emitted. Which code correctly implements this behavior?

hard

A. class PingCounter extends EventEmitter {
constructor() {
super();
this.count = 0;
this.on('ping', () => this.count++);
}
}

B. class PingCounter extends EventEmitter {
constructor() {
this.count = 0;
this.on('ping', () => this.count++);
super();
}
}

C. class PingCounter extends EventEmitter {
count = 0;
on('ping', () => this.count++);
}

D. class PingCounter extends EventEmitter {
constructor() {
super();
this.count = 0;
this.emit('ping', () => this.count++);
}
}

Solution

1. Step 1: Proper constructor and super() call

   class PingCounter extends EventEmitter { constructor() { super(); this.count = 0; this.on('ping', () => this.count++); } } correctly calls super() first in constructor, required before using this.

2. Step 2: Correct event listener setup

   class PingCounter extends EventEmitter { constructor() { super(); this.count = 0; this.on('ping', () => this.count++); } } uses this.on('ping', () => this.count++) to increment count on each ping event.

3. Step 3: Check other options for errors

   class PingCounter extends EventEmitter { constructor() { this.count = 0; this.on('ping', () => this.count++); super(); } } calls this.on before super(), causing error. class PingCounter extends EventEmitter { count = 0; on('ping', () => this.count++); } has invalid syntax outside constructor. class PingCounter extends EventEmitter { constructor() { super(); this.count = 0; this.emit('ping', () => this.count++); } } wrongly uses emit instead of on.

4. Final Answer:

   class PingCounter extends EventEmitter { constructor() { super(); this.count = 0; this.on('ping', () => this.count++); } } -> Option A

5. Quick Check:

   super() first, then on() listener = A [OK]

Hint: Always call super() before using this in constructor [OK]

Common Mistakes:

• Calling this before super() in constructor
• Using emit() instead of on() to listen
• Placing on() calls outside constructor or methods