javascript延迟加载外部js文件

function _GetJsData(url, callback) { 
    var scripts = document.createElement('script'); 
    document.body.appendChild(scripts); 
 
    scripts.onload = function() { 
        isok(); 
       // document.body.removeChild(this); 
    }; 
    scripts.onreadystatechange = function() { 
        if (this.readyState == "loaded") { 
            isok(); 
        //    document.body.removeChild(this); 
        } 
    }; 
    scripts.charset = "GBK"; 
    scripts.src = url; 
} 
_GetJsData('http://i03.youxigu.com/js/util/yahoo.js','isok'); 
var isok=function(){alert('isok')};