<html><head>
<script>
var counter = 0;
function increaseCounter() {
++counter;
alert("Counter is " + counter);
}
</script>
<script>
var counter="A";
</script>
</head><body><form>
<input type="button" value="Increase Counter" onclick="increaseCounter();" />
</form></body></html>
will output "Counter is NaN" when the button is clicked.
Global variables in JavaScript can be eliminated by closures. For example the previous JavaScript can be refactored as,
<html><head>
<script>
var CounterJS = (function() {
var counter = 0;
return {
increaseCounter: function() {
++counter;
alert("Counter is " + counter);
}
}
}());
</script>
<script>
var counter="A";
</script>
</head><body><form>
<input type="button" value="Increase Counter" onclick="CounterJS.increaseCounter();" />
</form></body></html>
