Object.freeze

let movie1 = {
  name: 'Star Wars',
  episode: 7
};
​
let movie2 = Object.freeze(Object.assign({}, movie1));
​
movie2.episode = 8; // fails silently in non-strict mode,
                    // throws error in strict mode
​
console.log(movie1.episode); // writes 7
console.log(movie2.episode); // writes 7