The JavaScript seal() 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, and existing properties can’t be removed, but existing properties can be changed.

Example:

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

Similar to Object.freeze() but does not make properties non-writable. In only prevents to add or remove properties.

Similar to Object.preventExtensions() but also disallows removing properties:

const dog = {}
dog.breed = 'Siberian Husky'
dog.name = 'Roger'
Object.seal(dog)
delete dog.name //TypeError: Cannot delete property 'name' of #<Object>