Object.seal

by Joshua McNeese

JavaScript

var obj = {
    prop: function () {},
    foo: "bar"
  };

// New properties may be added, existing properties may be changed or removed
obj.foo = "baz";
obj.lumpy = "woof";
delete obj.prop;

var o = Object.seal(obj);

console.log(o === obj);
console.log(Object.isSealed(obj) === true);

// Changing property values on a sealed object still works.
obj.foo = "quux";

// But you can't convert data properties to accessors, or vice versa.
Object.defineProperty(obj, "foo", { get: function() { return "g"; } }); // throws a TypeError

// Now any changes, other than to property values, will fail.
obj.quaxxor = "the friendly duck"; // silently doesn't add the property
delete obj.foo; // silently doesn't delete the property

// ...and in strict mode such attempts will throw TypeErrors
function fail() {
  "use strict";
  delete obj.foo; // throws a TypeError
  obj.sparky = "arf"; // throws a TypeError
}
fail();

// Attempted additions through Object.defineProperty will also throw
Object.defineProperty(obj, "ohai", { value: 17 }); // throws a TypeError
Object.defineProperty(obj, "foo", { value: "eit" }); // changes existing property value