js的格式有关问题！

js的格式问题！~~~~~~！！
想问问最前面加个“;”是干嘛用？？！！

JScript code

;(function($) {    var defaults = {        mouseOutOpacity:   0.67,        mouseOverOpacity:  1.0,        fadeSpeed:         'fast',        exemptionSelector: '.selected'    };    $.fn.opacityrollover = function(settings) {        // Initialize the effect        $.extend(this, defaults, settings);        var config = this;        function fadeTo(element, opacity) {            var $target = $(element);                        if (config.exemptionSelector)                $target = $target.not(config.exemptionSelector);                            $target.fadeTo(config.fadeSpeed, opacity);        }        this.css('opacity', this.mouseOutOpacity)            .hover(                function () {                    fadeTo(this, config.mouseOverOpacity);                },                function () {                    fadeTo(this, config.mouseOutOpacity);                });        return this;    };})(jQuery);