// New objects are extensible.
var empty = {};
Object.isExtensible(empty); // === true
// ...but that can be changed.
Object.preventExtensions(empty);
Object.isExtensible(empty); // === false
// Sealed objects are by definition non-extensible.
var sealed = Object.seal({});
Object.isExtensible(sealed); // === false
// Frozen objects are also by definition non-extensible.
var frozen = Object.freeze({});
Object.isExtensible(frozen); // === false
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