HowTo set a global variable in javascript or jQuery

hi all, Ive read alot on the web that simply doesnt make sense on this hoping to have a clear simple discussion on how to handle a string and perform comparison against it along the open session...

e.g.
top of page something like setting default state like so:
<script language="javascript">
var globalSessionValue="false";
</script>
... then many MANY lines later a later function changes the value of the string like so:
<script language="javascript">
function whatever(){
if ( getElementbyID("WhateveruserEvent").value ... whatever comparison here <= === ! > 0) {
globalSessionValue="true";
}
</script>
...
so this way later I can perform another simple if/else on the changed state

<script type="text/javascript">
function evaluateRestriction(){ // this has tested to fire On TAB into the field CONFIRMED
var checkState = globalSessionValue.value; 
		if ( globalSessionValue.length > 4) { //false
			document.getElementById("LogicContainer").innerHTML = "<img src='SHOW GOOD PHOTO' />";
		} else { 
			document.getElementById("LogicContainer").innerHTML = ""; // DONT SHOW anything here

		}	
	}
</script>

<div id="LogicContainer">

</div>

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