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Node.js Design Patterns

The Observer Pattern: Observable Objects

Explore how to implement the Observer pattern by extending the EventEmitter class to make any object observable in Node.js. Understand how to manage event listeners to prevent memory leaks and ensure efficient resource handling in asynchronous applications.

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Making any object observable

In the Node.js world, the EventEmitter class is rarely used on its own. Instead, it’s more common to see it extended by other classes. In practice, this enables any class to inherit the capabilities of the EventEmitter class, therefore becoming an observable object.

To demonstrate this pattern, let’s try to implement the functionality of the findRegex() function in a class, as follows:

Node.js
import { EventEmitter } from 'events'
import { readFile } from 'fs'
class FindRegex extends EventEmitter {
  constructor (regex) {
    super()
    this.regex = regex
    this.files = []
  }
  addFile (file) {
    this.files.push(file)
    return this
  }
  find () {
    for (const file of this.files) {
      readFile(file, 'utf8', (err, content) => {
        if (err) {
          return this.emit('error', err)
        }
        this.emit('fileread', file)
        const match = content.match(this.regex)
        if (match) {
          match.forEach(elem => this.emit('found', file, elem))
        }
      }) 
    }
    return this 
  }
}

The FindRegex class that we just defined extends the EventEmitter class to become a fully-fledged observable class. Always remember to use the super() function in the constructor to initialize the EventEmitter internals. ...