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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Running sum of array</title>
</head>
<body>
  <h1>Running total</h1>
  <p>Edit <code>arr</code>, then <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      var arr = [10, -2, 7];
      var total = 0;
      var lines = [];
      var i;
      lines.push("Running total:");
      for (i = 0; i < arr.length; i++) {
        var value = arr[i];
        total += value;
        var signed = value >= 0 ? "+" + value : String(value);
        lines.push("  after " + signed + ": " + total);
      }
      lines.push("Final sum: " + total);
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

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JavaScript Sum of Array — Running Total (Try It)