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.

The Object.isSealed() static method determines if an object is sealed.

Try it

const object = {
  foo: 42,
};

console.log(Object.isSealed(object));
// Expected output: false

Object.seal(object);

console.log(Object.isSealed(object));
// Expected output: true

Syntax

Object.isSealed(obj)

Return value

A Boolean indicating whether or not the given object is sealed.

Returns true if the object is sealed, otherwise false. An object is sealed if it is not extensible and if all its properties are non-configurable and therefore not removable (but not necessarily non-writable).

Examples

Using Object.isSealed

js
// 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 a TypeError. In ES2015, it will return true without any errors if a non-object argument is passed, since primitives are, by definition, immutable.

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

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

Specifications

Specification

ECMAScript® 2027 Language Specification
# sec-object.issealed

Browser compatibility

Enable JavaScript to view this browser compatibility table.

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This page was last modified on Jul 20, 2025 by MDN contributors.

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Standard built-in objects
Object
Constructor
1. Object()
Static methods
Instance methods
Instance properties
1. __proto__
2. constructor
Inheritance
Object/Function
Static methods
Static properties
Instance methods
Instance properties
1. __proto__
2. constructor