JavaScript: Object Extensible and Prevent Modification

In JavaScript, every object has a special info called “extensible attribute”. It specifies whether properties can be added to the object.

By default, all native objects (⁖ array, function, date, regex, …) and user-defined objects are extensible. Host objects (⁖ document.) may or may not be extensible.

You can set a object to be not extensible, but you cannot set a object to be extensible.

Check If Object is Extensible

You can use Object.isExtensible(obj) to check if a object is extensible. Example:

// check if a object is extensible (that is, whether properties can be added)

var uu = {}; // user-defined object
console.log(Object.isExtensible(uu)); // true

var aa = [3,4,5]; // Array
console.log(Object.isExtensible(aa)); // true

var dd = new Date(); // date object
console.log(Object.isExtensible(dd)); // true

Prevent Extension

You can set the extensible attribute to false using the method Object.preventExtensions(obj).

// set the extensible attribute of a object to false

var u8 = {};

Object.preventExtensions(u8);

u8.pp = 3;                      // add a property

console.log(u8.pp);             // prints 「undefined」. add failed

console.log(Object.isExtensible(u8)); // false

Note: Object.preventExtensions(obj) set a object's extensible attribute to false. It does not change its parent's extensible attribute. This means, if someone added a property to some object that's in your object's prototype chain, you might get unexpected properties due to inheritance.

Note: there's no way to set a object's extensible attribute to true.

〔☛ JavaScript Tutorial: Understanding Prototype ＆ Inheritance〕

「Object.seal()」 ≈ Prevent Extension and Make ALL Properties Non-configurable

Object.seal(obj) will do 2 things:

• Set the object's extension attribute to false.
• Set ALL of the object's own property's “configurable” attributes to false.

(note: “configurable” is a attribute of property. When false, it means that property cannot be deleted (⁖ delete people.john) and this property's attributes {writable, enumerable, configurable} cannot be changed.)

// seal a object.

var oo = {"p1":1, "p2":2};

console.log(Object.getOwnPropertyDescriptor(oo, "p1"));
// → { value: 1, writable: true, enumerable: true, configurable: true }

Object.seal(oo);

// configurable attribute for property p1 is now false
console.log(Object.getOwnPropertyDescriptor(oo, "p1"));
// → { value: 1, writable: true, enumerable: true, configurable: false }

// configurable attribute for property p2 is now false
console.log(Object.getOwnPropertyDescriptor(oo, "p2"));
// → { value: 2, writable: true, enumerable: true, configurable: false }

// Extensible attribute is now false
console.log(Object.isExtensible(oo)); // false

Check If Object is Sealed

Use Object.isSealed() to check if a object is sealed.

var oo = {"p1":1, "p2":2};
console.log(Object.isSealed(oo)); // false
Object.seal(oo);
console.log(Object.isSealed(oo)); // true

「Object.freeze()」 = 「Object.seal()」 + Make ALL Properties Non-writable

Object.freeze(obj) will do 3 things:

• Set the object's extension attribute to false.
• Set ALL of the object's own property's “configurable” attributes to false.
• Set ALL of the object's own property's “writable” attributes to false.

When a property's “writable” attribute is false, you cannot change the property's value.

// freeze a object.

var oo = {"p1":1, "p2":2};

console.log(Object.getOwnPropertyDescriptor(oo, "p1"));
// → { value: 1, writable: true, enumerable: true, configurable: true }

Object.freeze(oo);

console.log(Object.getOwnPropertyDescriptor(oo, "p1"));
// → { value: 1, writable: false, enumerable: true, configurable: false }

console.log(Object.getOwnPropertyDescriptor(oo, "p2"));
// → { value: 2, writable: false, enumerable: true, configurable: false }

// Extensible attribute is now false
console.log(Object.isExtensible(oo)); // false

Check If Object is Frozen

Use Object.isFrozen() to check if a object is frozen.

var oo = {"p1":1, "p2":2};
console.log(Object.isFrozen(oo)); // false
Object.freeze(oo);
console.log(Object.isFrozen(oo)); // true