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')