Object.isFrozen()

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

Syntax

Object.isFrozen(obj)

Parameters

obj

The object which should be checked.

Return value

A Boolean indicating whether or not the given object is frozen.

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()