const birthDateHandler = {
    get: (target, property) => {
      if (property === "birthDate"){
        const currentYear = new Date().getFullYear()

        return currentYear - target.age
      }

      const propertyExists = property in target

      return propertyExists
        ? target[property]
        : new Error(`the property ${property} does not exist!`)
    }
}

user = new Proxy(user, birthDateHandler)

console.log(user.birthDate)
//output: `${user.birthDate}`