Object.isFrozen()

Summary

The Object.isFrozen() determines if an object is frozen.

Syntax

Object.isFrozen(obj)

Parameters

obj
The object which should be checked.

Description

An object is frozen if and only if it is not extensible, all its properties are non-configurable, and all its data properties (that is, properties which are not accessor properties with getter or setter components) are non-writable.

Examples

// A new object is extensible, so it is not frozen.
assert(Object.isFrozen({}) === false);

// An empty object which is not extensible is vacuously frozen.
var vacuouslyFrozen = Object.preventExtensions({});
assert(Object.isFrozen(vacuouslyFrozen) === true);

// A new object with one property is also extensible, ergo not frozen.
var oneProp = { p: 42 };
assert(Object.isFrozen(oneProp) === false);

// Preventing extensions to the object still doesn't make it frozen,
// because the property is still configurable (and writable).
Object.preventExtensions(oneProp);
assert(Object.isFrozen(oneProp) === false);

// ...but then deleting that property makes the object vacuously frozen.
delete oneProp.p;
assert(Object.isFrozen(oneProp) === true);

// A non-extensible object with a non-writable but still configurable property is not frozen.
var nonWritable = { e: "plep" };
Object.preventExtensions(nonWritable);
Object.defineProperty(nonWritable, "e", { writable: false }); // make non-writable
assert(Object.isFrozen(nonWritable) === false);

// Changing that property to non-configurable then makes the object frozen.
Object.defineProperty(nonWritable, "e", { configurable: false }); // make non-configurable
assert(Object.isFrozen(nonWritable) === true);

// A non-extensible object with a non-configurable but still writable property also isn't frozen.
var nonConfigurable = { release: "the kraken!" };
Object.preventExtensions(nonConfigurable);
Object.defineProperty(nonConfigurable, "release", { configurable: false });
assert(Object.isFrozen(nonConfigurable) === false);

// Changing that property to non-writable then makes the object frozen.
Object.defineProperty(nonConfigurable, "release", { writable: false });
assert(Object.isFrozen(nonConfigurable) === true);

// A non-extensible object with a configurable accessor property isn't frozen.
var accessor = { get food() { return "yum"; } };
Object.preventExtensions(accessor);
assert(Object.isFrozen(accessor) === false);

// ...but make that property non-configurable and it becomes frozen.
Object.defineProperty(accessor, "food", { configurable: false });
assert(Object.isFrozen(accessor) === true);

// But the easiest way for an object to be frozen is if Object.freeze has been called on it.
var frozen = { 1: 81 };
assert(Object.isFrozen(frozen) === false);
Object.freeze(frozen);
assert(Object.isFrozen(frozen) === true);

// By definition, a frozen object is non-extensible.
assert(Object.isExtensible(frozen) === false);

// Also by definition, a frozen object is sealed.
assert(Object.isSealed(frozen) === true);

Specifications

Specification Status Comment
ECMAScript Language Specification 5.1th Edition (ECMA-262) Standard Initial definition.
Implemented in JavaScript 1.8.5
ECMAScript Language Specification 6th Edition (ECMA-262) Draft  

Browser compatibility

Feature Firefox (Gecko) Chrome Internet Explorer Opera Safari
Basic support 4 (2.0) 6 9 Not supported 5.1
Feature Firefox Mobile (Gecko) Android IE Mobile Opera Mobile Safari Mobile
Basic support ? ? ? ? ?

Based on Kangax's compat table.

See also

Document Tags and Contributors

Contributors to this page: evilpie, Sheppy, trupanka, fscholz, dbruant, ziyunfei, ethertank, Waldo, Crash
Last updated by: fscholz,