问如何检查jquery是否已经加载?我们可以通过编程方式用新版本替换旧版本吗？

EN

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script>
    window.jQuery || document.write('<script src="<?php echo SITE_JS;?>/jquery-1.9.1.min.js"><\/script>');
 // you should have jquery-1.9.1.min.js on the site url
</script>

if (typeof jQuery != 'undefined') {  
        // jQuery is loaded => print the version
          alert(jQuery.fn.jquery);}

(function() {
if (window.addEventListener) {
    // Standard
    window.addEventListener('load', jQueryCheck, false);
}
else if (window.attachEvent) {
    // Microsoft
    window.attachEvent('onload', jQueryCheck);
}
function jQueryCheck() {
    if (typeof jQuery === "undefined") {
        // No one's loaded it; either load it or do without
    }
}
})();