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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Condense — reduction chain</title>
</head>
<body>
  <h1>Reduction chain</h1>
  <p>Edit <code>number</code>, then click <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      function condenseWithChain(number) {
        var n = number;
        var chain = [n];
        while (n > 9) {
          var digitSum = 0;
          while (n > 0) {
            digitSum += n % 10;
            n = Math.floor(n / 10);
          }
          n = digitSum;
          chain.push(n);
        }
        return { root: n, chain: chain };
      }
      var number = 9875;
      var result = condenseWithChain(number);
      var lines = [];
      lines.push(result.chain.join(" -> "));
      lines.push("Digital root: " + result.root);
      lines.push("Also: 1 + (" + number + " - 1) % 9 = " + (1 + (number - 1) % 9));
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

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JavaScript Condense a Number — reduction chain (Try It)