An example of an extensible interface

extensible.js

// not extensible

function getUser() {
  return {
    name: 'Justin',
    email: 'justinfuller@email.com'
  }
}

// Extensible

class User {
  constructor(options = {}) {
    this.userData = options
  }
  
  get() {
    return this.userData
  }
  
  set(key, value) {
    this.userData[key] = value
  }
}