Add a js file based ona variable

appendscript.js

 document.addEventListener("DOMContentLoaded", function() {
     var src, scriptTag;
     src = "./app.js"; // here goes the name of the file to include
     scriptTag = document.createElement('SCRIPT');
     scriptTag.src = src;
     document.body.appendChild(scriptTag);
 });
 
 

appendtoheadinstead

     var src, scriptTag;
     src = "./app.js"; // here goes the name of the file to include
     scriptTag = document.createElement('SCRIPT');
     scriptTag.src = src;
     document.getElementsByTagName('head')[0].appendChild(scriptTag);

writescripttagshitty.js

     var src;
     src = "./app.js"; // here goes the name of the file to include
     document.write('<script src="'+src+'"></script>');