callback();
};
script.src = url;
document.getElementsByTagName('head')[0].appendChild (script);
}
</script>
实例：
<script type="text/javascript">
// prevent google analytics from slowing down page loading
window.addEventListener ('load', function() {
loadScript ('http://www.google-analytics.com/urchin.js', function() {
window._uacct = 'UA-xxxxxx-x';
urchinTracker();
});
}, false);
</script>
