使用闭包实现私有变量

const User = (function () {
  let _password
  function User(username, password) {
    this.username = username
    _password = password
  }
  User.prototype.getInfo = function () {
    return `${this.username} ${_password}`
  }
  return User
})()

const u1 = new User('Jack', 123456)
console.log(u1.username) // Jack
console.log(u1.password) // undefined
console.log(u1._password) // undefined
console.log(u1.getInfo()) // Jack 123456