Object.isSealed

Object.isSealed(obj)

// Objects aren't sealed by default.
var empty = {};
Object.isSealed(empty // === false

// If you make an empty object non-extensible,
// it is vacuously sealed.
Object.preventExtensions(empty
Object.isSealed(empty // === true

// The same is not true of a non-empty object,
// unless its properties are all non-configurable.
var hasProp = { fee: 'fie foe fum' };
Object.preventExtensions(hasProp
Object.isSealed(hasProp // === false

// But make them all non-configurable
// and the object becomes sealed.
Object.defineProperty(hasProp, 'fee', {
  configurable: false
}
Object.isSealed(hasProp // === true

// The easiest way to seal an object, of course,
// is Object.seal.
var sealed = {};
Object.seal(sealed
Object.isSealed(sealed // === true

// A sealed object is, by definition, non-extensible.
Object.isExtensible(sealed // === false

// A sealed object might be frozen,
// but it doesn't have to be.
Object.isFrozen(sealed // === true 
// (all properties also non-writable)

var s2 = Object.seal{ p: 3 }
Object.isFrozen(s2 // === false 
// ('p' is still writable)

var s3 = Object.seal{ get p() { return 0; } }
Object.isFrozen(s3 // === true
// (only configurability matters for accessor properties)

Object.isSealed(1
// TypeError: 1 is not an object (ES5 code)

Object.isSealed(1
// true                          (ES2015 code)