t.js

"use strict";

function loader() Components.classes["@mozilla.org/moz/jssubscript-loader;1"]  
                            .getService(Components.interfaces.mozIJSSubScriptLoader);  

var a = {};
Object.defineProperty(a, "foo", {value: 1, writable: false});

print("* directly assign");
try {
	var b = {__proto__: a};
	b.foo = 2;
	print(b.foo);
} catch(e) { print(e) }

print("* with subscript loader");
try {
	var b = {__proto__: a};
	io.writeFile("/tmp/tmp.js", "var foo = 2");
	loader().loadSubScript("file:///tmp/tmp.js", b);
	print(b.foo);
} catch(e) { print(e) }

print("* with defineProperty");
try {
	var b = {__proto__: a};
	Object.defineProperty(b, "foo", {value: 2, writable: true});
	print(b.foo);
	b.foo = 3;
	print(b.foo);
} catch(e) { print(e) }

print("* with __defineGetter__");
try {
	var b = {__proto__: a};
	b.__defineGetter__("foo", function() 2);
	print(b.foo);
} catch(e) { print(e) }