You use a constructor function like this:

function DogConstructor(name) {
    this.name = name;
    this.respondTo = function(name) {
        if(this.name == name) {
            alert(“Woof”);        
        }
    };
}

var spot = new DogConstructor(“Spot”);
spot.respondTo(“Rover”); // nope
spot.respondTo(“Spot”); // yeah!

If you've attended any of my recent MSDN events, here's a link to the OOP Javascript article that I recommended - it's a great read! 

Link to JavaScript: Create Advanced Web Applications With Object-Oriented Techniques -- MSDN Magazine, May 2007