semi-private-variables.js

function User(username) {
  const secretPasswordKey = Symbol('password');
  
  this.username = username;
  
  this.setPassword = function setPassword(password) {
    this[secretPasswordKey] = password;
  };
  
  this.authenticate = function authenticate(attempt) {
    return attempt === this[secretPasswordKey];
  };
}

let u = new User('jbfr');
u.setPassword('myPassword');

console.log(u['secretPasswordKey']);            // null
console.log(u[Symbol('secretPasswordKey')]);    // null
console.log(u.authenticate('badPassword'));     // false
console.log(u.authenticate('myPassword'));      // true