function load_script(xyUrl, callback){

    var head = document.getElementsByTagName('head')[0];

    var script = document.createElement('script');

    script.type = 'text/javascript';

    script.src = xyUrl;

    //借鉴了jQuery的script跨域方法

    script.onload = script.onreadystatechange = function(){

        if((!this.readyState || this.readyState === "loaded" || this.readyState === "complete")){

            callback && callback();

            // Handle memory leak in IE

            script.onload = script.onreadystatechange = null;

            if ( head && script.parentNode ) {

                head.removeChild( script );

            }

        }

    };

    // Use insertBefore instead of appendChild  to circumvent an IE6 bug.

    head.insertBefore( script, head.firstChild );

}