js按需加载兼容性写法

demo.js

var tools={
  test:function(){
    console.log(a)
  },
  demo:function(){
    
  }
}

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


loadscript('demo.js','test')