Private properties with no memory leaks.

privates.js

var Thing = (function(){

    var instances = []
      , data = []

    function setId(){
        data[instances.indexOf(this)] = id
    }

    function getId(){
        return data[instances.indexOf(this)]
    }

    function Thing(id){
        instances.push(this)
        setId.call(this, id)
    }

    Thing.prototype.destroy = function(){
        var index = instances.indexOf(this)
        instances.splice(index, 1)
        data.splice(index, 1)
    }

    Thing.prototype.someMethod = function(){
        var id = getId.call(this)
        // use id for whatever
    }

    return Thing

 }())