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<!DOCTYPE html>
<html lang=en>
<head>
  <meta charset=utf-8>
  <title>Sum of digits input example</title>
</head>
<body>
  <h1>Sum of digits (input style)</h1>
  <pre id=out></pre>
  <script>
    (function () {
      function parseInteger(text) {
        var t = String(text || "").trim();
        if (!/^-?\d+$/.test(t)) return null;
        var n = Number(t);
        if (!Number.isSafeInteger(n)) return null;
        return n;
      }
      function digitSum(rawInput) {
        var text = String(rawInput || "").trim();
        if (!/^-?\d+$/.test(text)) return null;
        var negative = text[0] === "-";
        var digitsText = negative ? text.slice(1) : text;
        if (digitsText === "0") return 0;
        var sum = 0;
        var i;
        for (i = 0; i < digitsText.length; i++) {
          var value = Number(digitsText[i]);
          if (i === 0 && negative) value = -value;
          sum += value;
        }
        return sum;
      }
      var rawInput = "-802";
      var number = parseInteger(rawInput);
      var lines = [];
      if (number === null) {
        lines.push("Invalid input.");
      } else {
        lines.push("Input: " + number);
        lines.push("Sum of digits: " + digitSum(rawInput));
      }
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

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JavaScript Sum of Digits — Input Style (Try It)