Javascript optimizing the code

I have written the following Javascript class

function InScroll( container, options ) {
    "use strict";

    var isRunning = false;
    // utilities
    var noop = function() {};

    var inter = null;

    if(!container) {
        alert('container element not provided');
        return;
    }
    if(!options.interval) options.interval = 500;

    // save screen info
    var page = {
        contentHeight: $(container).getHeight(),
        pageHeight: document.viewport.getHeight(),
        scrollPosition : 0,
    }

    function scroll() {
        var pos = $(container).select("div.kurz").length;

        if(pos == 0) {
            this.kill();
            return;
        }

        // get scroll position y-axis.
        page.scrollPosition = document.viewport.getScrollOffsets()[1];

        if( !isRunning && (page.contentHeight - page.pageHeight - page.scrollPosition ) < 450) {        
            new Ajax.Request(options.url + "?pos=" + pos, {
                onCreate: onCreate,
                onSuccess: append,
                onFailure: error,
                onComplete: onComplete

            });
        }
    }
    function onCreate() {
        isRunning = true;
    }

    function onComplete() {
        isRunning = false;
        page.contentHeight= $(container).getHeight();

    }

    function append( response ) {
        var resp = response.responseText.strip();

        if(resp == "") kill();

        container.innerHTML += resp;
    }

    function error( response ) {
        var resp = response.responseText.strip();
        container.innerHTML += resp;
    }

    function start() {

        if(!container) {
            alert('container element not provided');
            return;
        }

        if(!options || !options.url) {
            alert('content loader script should be set as options.url');
            return;
        }

        (function(that) {
            inter = window.setInterval( function() { that.scroll()}, 50);
        })(this)
    }

    function kill() {
        clearInterval(inter);
        $(options.loader).hide();
    }

    return {
        start: start,
        kill : kill,
        scroll: scroll
    }
}

Here I wanted to expose only the start and kill functions but due to the use of windows.setInterval i had to expose also the scroll function.

Can someone please have a look at the code, and give some suggestion how may I optiomize it?