The Object.isExtensible() static 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 their <span class="createlink">Prototype</span> 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)