The Object.isSealed() static 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
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 a TypeError. In ES2015, it will return true 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)