JavaScript Object.isExtensible()

In this tutorial, you will learn about the JavaScript Object.isExtensible() method with the help of examples.

The Object.isExtensible() method checks if an object is extensible i.e. new properties can be added to it.


// new objects are extensible
let empty = {};

// Output: true

isExtensible() syntax

The syntax of the isExtensible() method is:


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

// Output: true

// prevent extenstion of obj

// check if obj is extensible or not

// 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.

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