The JavaScript preventExtensions() method of the Object object takes an object as argument, and returns the same object. The object passed as argument is mutated and it’s now an object that will not accept new properties. New properties can’t be added, but existing properties can be removed, and existing properties can be changed.

Example:

const dog = {}
dog.breed = 'Siberian Husky'
Object.preventExtensions(dog)

dog.name = 'Roger' //TypeError: Cannot add property name, object is not extensible

The argument passed as argument is also returned as argument, hence dog === myDog (it’s the same exact object).

We can’t add new properties, but we can remove existing properties:

const dog = {}
dog.breed = 'Siberian Husky'
dog.name = 'Roger'
Object.preventExtensions(dog)
delete dog.name
dog //{ breed: 'Siberian Husky' }