jquery position scroll div fixed and height fix and bottom remove

HTML

<div id="f">
    I'm going to follow you only so far...
</div>

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JS 

var windw = this;

$.fn.followTo = function(pos) {
    var $this = this,
        $window = $(windw);

    $window.scroll(function(e) {
        if ($window.scrollTop() > pos) {
            $this.css({
                position: 'absolute',
                top: pos
            });
        } else {
            $this.css({
                position: 'fixed',
                top: 0
            });
        }
    });
};

$('#f').followTo(250);


CSS

#f {
    background:#ace;
    padding:20px;
    width:200px;
    position:fixed; top:0; left:0;
}