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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).