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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Trace digit sum</title>
</head>
<body>
  <h1>Trace digit peeling</h1>
  <p>Edit <code>n</code>, then <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      var n = -802;
      var x = Math.abs(n);
      var total = 0;
      var lines = [];
      lines.push("Starting from Math.abs(" + n + ") = " + x);
      while (x !== 0) {
        var digit = x % 10;
        total += digit;
        lines.push("  take " + digit + ", remaining " + Math.floor(x / 10) + ", total=" + total);
        x = Math.floor(x / 10);
      }
      lines.push("Digit sum: " + total);
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

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JavaScript Sum of Digits — Trace -802 (Try It)