Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<p>Counting with a global variable.</p>

<button type="button" onclick="myFunction()">Count!</button>

<p id="demo">0</p>

<script>
var counter = 0;

function add() {
    return counter += 1;
}

function myFunction(){
    document.getElementById("demo").innerHTML = add();
}
</script>

</body>
</html>