JavaScript Object.isExtensible()

// new objects are extensible
let empty = {};
console.log(Object.isExtensible(empty)); 

// Output: true

isExtensible() syntax

The syntax of the isExtensible() method is:

Object.isExtensible(obj)

Here, isExtensible() is a static method. Hence, we need to access the method using the class name, Object.

isExtensible() Parameters

The isExtensible() method takes in:

• obj - the object which should be checked.

isExtensible() Return Value

The isExtensible() method returns a Boolean value:

• true - if object is extensible
• false - if object is not extensible

Note: An object can be marked as non-extensible using Object.preventExtensions(), Object.seal(), or Object.freeze().

Example: JavaScript Object.isExtensible()

// create an empty object
let obj = {};

// new objects are extensible
console.log(Object.isExtensible(obj)); 

// Output: true

// prevent extenstion of obj
Object.preventExtensions(obj);

// check if obj is extensible or not
console.log(Object.isExtensible(obj));

// Output: false 

In the above example, we have created an empty object obj and checked its extensibility using the isExtensible() method.

By default, new objects are extensible: we can add new properties to them.

However, we can prevent an object from being extended using the preventExtensions() method. When we call this method, the object becomes non-extensible.

When we check for the extensibility of obj again, we get false as an output, indicating that the object is no more extensible.

Note: Sealed and frozen objects are non-extensible by definition.

Recommended Reading:

• JavaScript Object.preventExtensions()