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<!DOCTYPE html>
<html lang=en>
<head>
  <meta charset=utf-8>
  <title>Happy number — single</title>
</head>
<body>
  <h1>Happy number check</h1>
  <p>Edit <code>number</code>, then <strong>Run</strong>.</p>
  <pre id=out></pre>
  <script>!function(){function t(t){for(var r=0;t>0;){var n=t%10;r+=n*n,t=Math.floor(t/10)}return r}var r=function(r){for(var n={},e=r;1!==e;){if(n[e])return!1;n[e]=!0,e=t(e)}return!0}(19)?"19 is a Happy Number.":"19 is not a Happy Number.";document.getElementById("out").textContent=r}()</script>
</body>
</html>

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JavaScript Happy Number — Single check (Try It)