Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<h1>With "use strict":</h1>
<h3>Using a variable without declaring it, is not allowed.</h3>

<p>Activate debugging in your browser (F12) to see the error report.</p>

<script>
"use strict";
x = 3.14;    // This will cause an error (x is not defined).
</script>

</body>
</html>