Object.isExtensible()
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The Object.isExtensible() static method determines if an object
is extensible (whether it can have new properties added to it).
Try it
const object = {};
console.log(Object.isExtensible(object));
// Expected output: true
Object.preventExtensions(object);
console.log(Object.isExtensible(object));
// Expected output: false
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 their [[Prototype]] can be re-assigned. An object can be marked as non-extensible using one of Object.preventExtensions(), Object.seal(), Object.freeze(), or Reflect.preventExtensions().
Examples
Using Object.isExtensible
// New objects are extensible.
const empty = {};
Object.isExtensible(empty); // true
// They can be made un-extensible
Object.preventExtensions(empty);
Object.isExtensible(empty); // false
// Sealed objects are by definition non-extensible.
const sealed = Object.seal({});
Object.isExtensible(sealed); // false
// Frozen objects are also by definition non-extensible.
const frozen = Object.freeze({});
Object.isExtensible(frozen); // false
Non-object argument
In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES2015, it will return false without any errors if a non-object argument is passed, since primitives are, by definition, immutable.
Object.isExtensible(1);
// TypeError: 1 is not an object (ES5 code)
Object.isExtensible(1);
// false (ES2015 code)
Specifications
| Specification |
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| ECMAScript® 2027 Language Specification # sec-object.isextensible |
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