object-methods-4.js

var user = {};

Object.defineProperty(user, 'age', {
  enumerable: false,                    // will make age not appear in Object.keys(user)
  configurable: false,                  // prevent deletion/ modification of the property and its value
  writable: false,                      // will prevent changing of the property value
  value: 30
});

Object.keys(user);
// return []
Object.getOwnPropertyNames(user);
// return ["age"]

user.age = 25;
// will throw "TypeError: Cannot assign to read only property 'age' of #<Object>" in strict mode
console.log(user.age);
// will log 30 as the property is not writable

delete user.age
// returns false