Dissecting jQuery's Fade Animation

fade.js

function $(id) {

    var $ = document.getElementById(id);
 
    var opacityTo = function (elm, v) {
        elm.style.opacity = v/100; // CSS3
        elm.style.MozOpacity =  v/100; // FF
        elm.style.KhtmlOpacity =  v/100; // Safari
        elm.style.filter=" alpha(opacity ="+v+")"; // IE
    };

    $.fade = function (delay, callbk, out) {
        var _this = this;

        // for ie
        _this.style.zoom = 1;
        _this.style.display="block";

        for (var i = 1; i <= 100; i++) {
            (function(j) {
                setTimeout(function() {
                    if (out == true) {
                        j = 100 - j;
                     }

                     opacityTo(_this, j);

                     if (j == 100 && callbk != undefined) { 
                        callbk.call(_this);
                     }
                     else if (out == true && callbk != undefined && j == 0) {
                        callbk.call(_this);
                     } 
                }, j*delay/100);
            })(i);
        }
    };

    $.fadein : function(elm, delay, callbk) {
            $.fade(elm, delay, callbk, false);
    };

    $.fadeout : function(elm, delay, callbk) {
            $.fade(elm, delay, callbk, true);
    };

    return $;
})();

Usage:
$("imgid").fadeIn(1000);