Can Read-Only Properties be Implemented in Pure JavaScript?

With any javascript interpreter that implements ECMAScript 5 you can use Object.defineProperty to define readonly properties. In loose mode the interpreter will ignore a write on the property, in strict mode it will throw an exception.

Example from ejohn.org:

var obj = {};
Object.defineProperty( obj, "<yourPropertyNameHere>", {
  value: "<yourPropertyValueHere>",
  writable: false,
  enumerable: true,
  configurable: true
});

Edit: Since this answer was written, a new, better way using Object.defineProperty has been standardized in EcmaScript 5, with support in newer browsers. See Aidamina's answer. If you need to support "older" browsers, you could use one of the methods in this answer as a fallback.

In Firefox, Opera 9.5+, and Safari 3+, Chrome and IE (tested with v11) you can define getter and setter properties. If you only define a getter, it effectively creates a read-only property. You can define them in an object literal or by calling a method on an object.

var myObject = {
    get readOnlyProperty() { return 42; }
};

alert(myObject.readOnlyProperty); // 42
myObject.readOnlyProperty = 5;    // Assignment is allowed, but doesn't do anything
alert(myObject.readOnlyProperty); // 42

If you already have an object, you can call __defineGetter__ and __defineSetter__:

var myObject = {};
myObject.__defineGetter__("readOnlyProperty", function() { return 42; });

Of course, this isn't really useful on the web because it doesn't work in Internet Explorer.

You can read more about it from John Resig's blog or the Mozilla Developer Center.