Object.isExtensible()
The Object.isExtensible()
method determines if an object
is extensible (whether it can have new properties added to it).
Syntax
Object.isExtensible(obj)
Parameters
obj
-
The object which should be checked.
Return value
A Boolean
indicating whether or not the given object is extensible.
Description
Objects are extensible by default: they can have new properties added to them, and (in
engines that support __proto__
) their
__proto__
property can be modified. An object can be marked as
non-extensible using Object.preventExtensions()
,
Object.seal()
, or Object.freeze()
.
Examples
Using Object.isExtensible
// New objects are extensible.
var empty = {};
Object.isExtensible(empty); // === true
// ...but that can be changed.
Object.preventExtensions(empty);
Object.isExtensible(empty); // === false
// Sealed objects are by definition non-extensible.
var sealed = Object.seal({});
Object.isExtensible(sealed); // === false
// Frozen objects are also by definition non-extensible.
var frozen = Object.freeze({});
Object.isExtensible(frozen); // === false
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 non-extensible ordinary object, return false
.
Object.isExtensible(1);
// TypeError: 1 is not an object (ES5 code)
Object.isExtensible(1);
// false (ES2015 code)
Specifications
Specification |
---|
ECMAScript Language Specification # sec-object.isextensible |
Browser compatibility
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