In raw javascript, you'll want to put an id on your anchor tag and do this:
<html>
<script>
var simpleText = "hello_world";
var finalSplitText = simpleText.split("_");
var splitText = finalSplitText[0];
function insertText(){
document.getElementById('someId').InnerHTML = splitText;}
</script>
<body onload="insertText()">
<a href = test.html id="someId">I need the value of "splitText" variable here</a>
</body>
</html>