The Object isSealed() method

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).