[jQuery] question about callbacks

[jQuery] question about callbacks

hi mates,
how do i implement the possibility to have a callback function in my plugin?
Via eval()?
jQuery.fn.setScrollableArea = function(modifier, callback){
    jQuery('body').css({
        overflow: 'hidden'
    });
    return this.each(function(){
        var offset = jQuery(this).offset();
        var availableHeight = jQuery(window).height();
        //alert("new height=" + availableHeight);
        modifier = (typeof modifier == 'undefined') ? 0 : modifier;
       
        var newHeight = availableHeight - offset.top - modifier;
        jQuery(this).css({
            height: newHeight,
            overflow: 'hidden'
        });
        callback();
    });
}
it seems to work, but i read somewhere eval() is evil() :)
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Alexandre