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
Using Object.isFrozen
// 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
Non-object coercion
In ES5, if the argument to this method is not an object (a primitive), then it will
cause a TypeError
. In ES2015, a non-object argument will be treated as if
it was a frozen ordinary object, return true
.
Object.isFrozen(1);
// TypeError: 1 is not an object (ES5 code)
Object.isFrozen(1);
// true (ES2015 code)
Specifications
Specification |
---|
ECMAScript Language Specification # sec-object.isfrozen |
Browser compatibility
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