Object.preventExtensions

by Joshua McNeese

JavaScript

// Object.preventExtensions returns the object being made non-extensible.
var obj = {};
var obj2 = Object.preventExtensions(obj);
console.log(obj === obj2);

// Objects are extensible by default.
var empty = {};
console.log(Object.isExtensible(empty) === true);

// ...but that can be changed.
Object.preventExtensions(empty);
console.log(Object.isExtensible(empty) === false);

// Object.defineProperty throws when adding a new property to a non-extensible object.
var nonExtensible = { removable: true };
Object.preventExtensions(nonExtensible);
Object.defineProperty(nonExtensible, "new", { value: 8675309 }); // throws a TypeError

// In strict mode, attempting to add new properties to a non-extensible object throws a TypeError.
function fail()
{
  "use strict";
  nonExtensible.newProperty = "FAIL"; // throws a TypeError
}
fail();