Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>
<p>If you use NaN in a mathematical operation, the result can be a concatination:</p>

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

<script>
var x = NaN;
var y = "5";
document.getElementById("demo").innerHTML = x + y;
</script>

</body>
</html>