Object.isSealed()

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

Syntax

Object.isSealed(obj)

Parameters

obj

The object which should be checked.

Return value

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

Description

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

// 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)

Notes

In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES6, a non-object argument will be treated as if it was a sealed ordinary object, simply return true.

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

Object.isSealed(1);
// true                          (ES6 code)

Specifications

Browser compatibility

See also

• Object.seal()
• Object.preventExtensions()
• Object.isExtensible()
• Object.freeze()
• Object.isFrozen()