target

The target object which to check if it is extensible.

A Boolean indicating whether or not the target is extensible.

TypeError

Thrown if target is not an object.

Reflect.isExtensible() provides the reflective semantic of checking if an object is extensible. The only difference with Object.isExtensible() is how non-object targets are handled. Reflect.isExtensible() throws a TypeError if the target is not an object, while Object.isExtensible() always returns false for non-object targets.

Reflect.isExtensible() invokes the [[IsExtensible]] object internal method of target.

See also Object.isExtensible().

// New objects are extensible.
const empty = {};
Reflect.isExtensible(empty); // true

// ...but that can be changed.
Reflect.preventExtensions(empty);
Reflect.isExtensible(empty); // false

// Sealed objects are by definition non-extensible.
const sealed = Object.seal({});
Reflect.isExtensible(sealed); // false

// Frozen objects are also by definition non-extensible.
const frozen = Object.freeze({});
Reflect.isExtensible(frozen); // false

If the target argument to this method is not an object (a primitive), then it will cause a TypeError. With Object.isExtensible(), a non-object target will return false without any errors.

Reflect.isExtensible(1);
// TypeError: 1 is not an object

Object.isExtensible(1);
// false

• Polyfill of Reflect.isExtensible in core-js
• Reflect
• Object.isExtensible()
• handler.isExtensible()