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

Example:

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

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