How to ensure that the JavaScript code on the page is fully loaded before executing other code

The following describes how to determine whether a Javascript file is fully loaded or whether all JS files on the page are fully loaded:

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);  

}  

How to make the script run in the order you set, using the nested method:

loadScript("file1.js",function(){  
      loadScript("file2.js",function(){  
            loadScript("file3.js",function(){  
                 alert("All files are loaded");  
            });  
      });  
});  

Of course, if you are familiar with jquery, the code is simpler:

$.getScript("file1.js", function(){
　　alert("Script loaded and executed.");
});