JavaScript .preventExtensions()

The Object.preventExtensions() method prevents new properties from being added to an object. Unlike Object.freeze() or Object.seal(), this method still allows existing properties to be modified or deleted. It returns the same object that was passed to it.

Note: Once an object is made non-extensible, it cannot be made extensible again. This operation is irreversible.

Syntax

Object.preventExtensions(obj)

Parameters:

• obj: The object to make non-extensible.

Return value:

Returns the same object that was passed in, now marked as non-extensible.

Example: Basic Usage of Object.preventExtensions()

The following code demonstrates how Object.preventExtensions() works:

const user = {
  name: 'Arthur',
  age: 42,
};

// Show object is extensible
console.log(Object.isExtensible(user));

// Prevent extensions
Object.preventExtensions(user);

// Show object is no longer extensible after using Object.preventExtensions()
console.log(Object.isExtensible(user));

// New properties cannot be added
user.email = '[email protected]';
console.log(user.email);

// Existing properties can be modified
user.age = 31;
console.log(user.age);

// Existing properties can be deleted
delete user.name;
console.log(user.name);

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The output generated by this code is:

true
false
undefined
31
undefined

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Codebyte Example

This example shows the effect of Object.preventExtensions() in non-strict mode, and notes how strict mode would throw a TypeError:

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