1.
function js_include(fileName){    
 document.write('<script type="text/javascript" src="' + fileName + '"></script>');
}

2.
document.scripts[0].src="newjs.js"

3.
<div id=test>script</div>
<script>
var S ='<span>&nbsp;</span><script DEFER>' + 'alert("hi");'+ '</script' + '>';
document.all.test.innerHTML = S;
</script>