Object.isSealed()
The Object.isSealed()
static method determines if an object is sealed.
Try it
Syntax
js
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
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 Language Specification # sec-object.issealed |
Browser compatibility
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