js的格式问题！~~~~~~！！

想问问最前面加个“;”是干嘛用？？！！;(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);