Object.isFrozen()

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.
Object.isFrozen({}); // === false

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

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

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

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

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

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

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

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

Notes

In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES6, a non-object argument will be treated as if it was a frozen ordinary object, simply return true.

Object.isFrozen(1);
// TypeError: 1 is not an object (ES5 code)

Object.isFrozen(1);
// true                          (ES6 code)

Specifications

Browser compatibility

See also

• Object.freeze()
• Object.preventExtensions()
• Object.isExtensible()
• Object.seal()
• Object.isSealed()