Prototypes & Classes

const animal = { eats: true }
const rabbit = { jumps: true }
Object.setPrototypeOf(rabbit, animal)
rabbit.jumps // true (own), rabbit.eats // true (prototype), rabbit.toString // Object.prototype

// Constructor function (pre-ES6)
function User(name, role) {
  this.name = name
  this.role = role
}
User.prototype.greet = function () {
  return "Hi, I'm " + this.name
}
new User("Alice", "admin").greet() // "Hi, I'm Alice"

// Class syntax (ES6 sugar)
class User {
  constructor(name, role) {
    this.name = name
    this.role = role
  }
  greet() {
    return "Hi, I'm " + this.name
  }
}

class Animal {
  constructor(name) {
    this.name = name
  }
  speak() {
    return this.name + " makes a sound"
  }
}
class Dog extends Animal {
  constructor(name, breed) {
    super(name)
    this.breed = breed
  }
  speak() {
    return this.name + " barks"
  }
}
new Dog("Rex", "Husky") instanceof Animal // true

class MathUtils {
  static clamp(v, min, max) {
    return Math.min(Math.max(v, min), max)
  }
}
MathUtils.clamp(15, 0, 10) // 10

class Account {
  #balance = 0
  constructor(v) {
    this.#balance = v
  }
  getBalance() {
    return this.#balance
  }
}
// acct.#balance // SyntaxError — truly private

const canFly = (obj) => ({ ...obj, fly: () => obj.name + " flies" })
const canSwim = (obj) => ({ ...obj, swim: () => obj.name + " swims" })
const duck = canSwim(canFly({ name: "Daffy", speak: () => "quacks" }))
duck.fly() // "Daffy flies"