The Object preventExtensions() method

Object.preventExtensions() makes an object non-extensible.

After that:

• you cannot add new own properties
• you cannot change the object’s prototype
• you can still change or delete existing properties when their descriptors allow it

The method mutates and returns the same object:

const dog = {
  breed: 'Siberian Husky'
}

const result = Object.preventExtensions(dog)

result === dog //true
Object.isExtensible(dog) //false

There is no way to make the object extensible again.

See the MDN Object.preventExtensions() reference and the ECMAScript specification for the complete behavior.

Adding a property fails

In strict mode, assigning a new property throws a TypeError:

'use strict'

const dog = {
  breed: 'Siberian Husky'
}

Object.preventExtensions(dog)
dog.name = 'Roger' //TypeError

Without strict mode, a normal property assignment fails silently. Object.defineProperty() throws in both modes when it tries to add a property.

Existing properties can still change

preventExtensions() does not freeze existing properties:

const dog = {
  breed: 'Siberian Husky',
  name: 'Roger'
}

Object.preventExtensions(dog)

dog.name = 'Syd'
delete dog.breed

dog //{ name: 'Syd' }

The normal writable and configurable descriptor rules still apply.

The prototype cannot change

Making an object non-extensible also fixes its current prototype:

const dog = Object.preventExtensions({})

Object.setPrototypeOf(dog, {
  speak() {
    return 'Hello'
  }
}) //TypeError

Use Object.seal() when you also want to prevent deleting or reconfiguring properties.

Use Object.freeze() when you additionally want to make existing data properties non-writable.

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