Since you only need it to work on the Mozilla platform, you can define a getter with no corresponding setter. The best way to do it is different for each of your examples.

In an object literal, there is a special syntax for it:

var XU = {

    get Cc() { return Components.classes; }

};

In your second exampe, you can use the __defineGetter__ method to add it to either aXU.prototype or to this inside the constructor. Which way is better depends on whether the value is different for each instance of the object.

Edit: To help with the readability problem, you could write a function like defineConstant to hide the uglyness.

function defineConstant(obj, name, value) {

    obj.__defineGetter__(name, function() { return value; });

}

Also, if you want to throw an error if you try to assign to it, you can define a setter that just throws an Error object:

function defineConstant(obj, name, value) {

    obj.__defineGetter__(name, function() { return value; });

    obj.__defineSetter__(name, function() {

        throw new Error(name +  is a constant);

    });

}

If all the instances have the same value:

function aXU() {

}



defineConstant(aXU.prototype, Cc, Components.classes);

or, if the value depends on the object:

function aXU() {

    // Cc_value could be different for each instance

    var Cc_value = return Components.classes;



    defineConstant(this, Cc, Cc_value);

}

For more details, you can read the Mozilla Developer Center documentation.