Object.isSealed()

Object.isSealed(obj)

// Objects aren't sealed by default.
const 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.
const 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.
const 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)

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

const 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)