The Object isFrozen() method

Accepts an object as argument, and returns true if the object is frozen, false otherwise. Objects are frozen when they are return values of the Object.freeze() function.

Example:

const dog = {}
dog.breed = 'Siberian Husky'
const myDog = Object.freeze(dog)
Object.isFrozen(dog) //true
Object.isFrozen(myDog) //true
dog === myDog //true

In the example, both dog and myDog are frozen. The argument passed as argument to Object.freeze() is mutated, and can’t be un-freezed. It’s also returned as argument, hence dog === myDog (it’s the same exact object).