Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<p>A function can access variables defined inside the function:</p>

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

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

<script>
function myFunction() {
    var a = 4;
    document.getElementById("demo").innerHTML = a * a;
} 

</script>

</body>
</html>