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
});