Object.freeze

Object.freeze(obj)

var obj = {
  prop: function() {},
  foo: 'bar'
};

// New properties may be added, existing properties may be
// changed or removed
obj.foo = 'baz';
obj.lumpy = 'woof';
delete obj.prop;

// Both the object being passed as well as the returned
// object will be frozen. It is unnecessary to save the
// returned object in order to freeze the original.
var o = Object.freeze(obj

o === obj; // true
Object.isFrozen(obj // === true

// Now any changes will fail
obj.foo = 'quux'; // silently does nothing
// silently doesn't add the property
obj.quaxxor = 'the friendly duck';

// In strict mode such attempts will throw TypeErrors
function fail(){
  'use strict';
  obj.foo = 'sparky'; // throws a TypeError
  delete obj.quaxxor; // throws a TypeError
  obj.sparky = 'arf'; // throws a TypeError
}

fail(

// Attempted changes through Object.defineProperty; 
// both statements below throw a TypeError.
Object.defineProperty(obj, 'ohai', { value: 17 }
Object.defineProperty(obj, 'foo', { value: 'eit' }

// It's also impossible to change the prototype
// both statements below will throw a TypeError.
Object.setPrototypeOf(obj, { x: 20 })
obj.__proto__ = { x: 20 }

// A frozen array is like a tuple.
let a=[0];
Object.freeze(a
// The array cannot be modified now.
a[0]=1;
a.push(2
// a=[0]
// A frozen array can be unpacked normally.
let b, c;
[b,c]=Object.freeze([1,2]
// b=1, c=2

obj1 = {
  internal: {}
};

Object.freeze(obj1
obj1.internal.a = 'aValue';

obj1.internal.a // 'aValue'

// To do so, we use this function.
function deepFreeze(obj) {

  // Retrieve the property names defined on obj
  var propNames = Object.getOwnPropertyNames(obj

  // Freeze properties before freezing self
  propNames.forEach(function(name) {
    var prop = obj[name];

    // Freeze prop if it is an object
    if (typeof prop == 'object' && prop !== null)
      deepFreeze(prop
  }

  // Freeze self (no-op if already frozen)
  return Object.freeze(obj
}

obj2 = {
  internal: {}
};

deepFreeze(obj2
obj2.internal.a = 'anotherValue';
obj2.internal.a; // undefined

> Object.freeze(1)
TypeError: 1 is not an object // ES5 code

> Object.freeze(1)
1                             // ES2015 code