javascript延迟加载的解决方案：

1.使用defer标签

<script type="text/javascript" src="load.js" defer></script>

?2.使用XMLHttpRequest对象加载

var xhr = new XMLHttpRequest();
xhr.open("get","load.js",true);
xhr.onreadystatechange=function(){
    if(readyState==4){
        if(status>=200&&status<300||status==304){
           var script = document.createElement("script");
            script.type = "text/javascript";
            script.src = xhr.responseText;
            document.body.appendChild(script);
        }
    }
}
xhr.send(null);

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3.使用推荐的无阻塞模式：

function loadScript（url , callback）{   
  
     var script = document.createElement("script");   
  
     script.type="text/javascript";   
  
     if(script.readyState){   
 
         script.onreadystatechange = function(){   
 
             if(script.readyState=="loaded"||script.readyState=="complete"){   
  
                  script.onreadystatechange=null;   
  
                  callback();   
  
            }   
  
         }   
  
     }else{   
  
        script.onload = function(){   
  
            callback();   
  
        }   
  
     }   
  
     script.src = url;   
  
     document.getElementsByName("head")[0].appendChild(script);   
  
}  

?4.HTML5中可以使用async方式设置异步加载js

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<script type="text/javascript" src="load.js" async="true"></script>

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关于JS异步加载的测试

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