Object.isSealed()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
TheObject.isSealed() static method determines if an object issealed.
Try it
const object = { foo: 42,};console.log(Object.isSealed(object));// Expected output: falseObject.seal(object);console.log(Object.isSealed(object));// Expected output: trueSyntax
Object.isSealed(obj)Parameters
objThe object which should be checked.
Return value
ABoolean indicating whether or not the given object is sealed.
Description
Returnstrue if the object is sealed, otherwisefalse. Anobject is sealed if it is notextensible andif all its properties are non-configurable and therefore not removable (but notnecessarily non-writable).
Examples
Using Object.isSealed
// 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)Non-object argument
In ES5, if the argument to this method is not an object (a primitive), then it will cause aTypeError. In ES2015, it will returntrue without any errors if a non-object argument is passed, since primitives are, by definition, immutable.
Object.isSealed(1);// TypeError: 1 is not an object (ES5 code)Object.isSealed(1);// true (ES2015 code)Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-object.issealed |