无阻塞加载js,防止因js加载不了影响页面显示的问题

var script = document.createElement ("script"); 
script.type = "text/javascript"; 
script.src = "file1.js";  
document.body.appendChild(script); 

function loadScript(url, callback){ 
  var script = document.createElement ("script") ; 
　 script.type = "text/javascript"; 
    
  if (script.readyState){ //IE 
    script.onreadystatechange = function(){ 
     if (script.readyState == "loaded" || script.readyState == "complete"){ 
      script.onreadystatechange = null; 
      callback();  
     } 
    }; 
   }  
   else { //Others 
    script.onload = function(){ callback(); 
   };  
  } 
  script.src = url; 
  document.getElementsByTagName("head")[0].appendChild(script);  
 } 
 
loadScript("file1.js", function(){ //调用 
  alert("File is loaded!");  
});