Encapsulation in JS

no_private_state.js

class Person {
  constructor(age) {
    this.age = age;
  }
}

let person = new Person(15);
person.age = -33;

// This lack of encapsulation and privacy is bad since it makes 
// it easy to use an object in an illegal way.
//
// The result will be more frequent runtime bugs and more defensive programming
// to account for undesirable state

private_state_with_factory_function.js

// Consider this alternative

function Person(age) {

  // Implicit private state from params, 
  // or pass in a state object and track it here as a var.

  // Hoise your public API up here (just using ES6 shorthand here)
  return {
    incrementAge,
    getAge
  };
  
  // Implementation of your API down here
  function incrementAge() {
    age++;
  }
  
  function getAge() {
    return age;
  }
}

let person = Person(15);

person.incrementAge();
person.getAge(); // 16
person.age // undefined

// Unfortunately, you can still do this:
person.age = -33;
person.age; // 33;

// But while the above is still illegal usage, 
// at least this doesn't affect the actual internal state referenced by the API:
person.getAge(); // still 16.