You need to declare the variable before you include the helpers.js file. Simply create a script tag above the include for helpers.js and define it there.
<script type="text/javascript" >
var myFunctionTag = false;
</script>
<script type="text/javascript" src="https://stackoverflow.com/questions/2932782/js/helpers.js"></script>
...
<script type="text/javascript" >
// rest of your code, which may depend on helpers.js
</script>