Node.js Fundamentals - Event Module

Key Concepts

• Emitter: Object that emits events.
• Listener: Function that runs when event is emitted.
• Events: Named signals (strings like “click” or “data”).
• Once vs.On: on listens everytime whereas once listens only once.

Common Commands

Import and create an instance as:

import { EventEmitter } from 'events'
 
const myEmitter = new EventEmitter()

1. Listen for an Event:

myEmitter.on('greet', (name) => {
  console.log(`Hello, ${name}`)
})

• Explanation: Sets up listener for “greet” event.

2. Emit and Event:

myEmitter.emit('greet', 'John')

• Explanation: Triggers the event, passing “John” to listener. Outputs: “Hello, John”

3. Listen Once:

myEmitter.once('once-event', () => {
  console.log('This runs only once')
})
 
myEmitter.emit('once-event') // Runs
myEmitter.emit('once-event') // Ignored

• Explanation: Stops listening.

4. Remove Listener:

function listener() {
  console.log('Event')
}
 
myEmitter.on('event', listener)
myEmitter.removeListener('event', listener)

• Explanation: Stops listening.

Events are async and non-blocking.