after.js

// User.js

const LEGAL_AGE = 21;

export const isMajor = (user) => {
  return user.age >= LEGAL_AGE;
};

// usage
import * as User from './User.js';

const user = { age: 21 };
User.isMajor(user); // true

// no problem if it's spread
const user2 = { ...user, age: 20 };
User.isMajor(user2); // false

before.js

// User.js

export default class User {
  static LEGAL_AGE = 21;

  constructor({ age }) {
    this.age = age;
  }
  
  isMajor() {
    return this.age >= User.LEGAL_AGE;
  }
}

// usage
import User from './User.js';

const user = new User({ age: 21 });
user.isMajor(); // true

// if spread, loses the reference for the class
const user2 = { ...user, age: 20 };
user2.isMajor(); // Error: user2.isMajor is not a function