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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Binary — manual place value</title>
</head>
<body>
  <h1>Manual place value</h1>
  <p>Edit <code>binaryNumber</code> (only 0 and 1 digits), then click <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      function binaryToDecimalManual(bits) {
        bits = bits.trim();
        if (!bits || /[^01]/.test(bits)) {
          throw new Error("Binary string must contain only 0 and 1");
        }
        var total = 0;
        var power = 0;
        for (var i = bits.length - 1; i >= 0; i--) {
          if (bits[i] === "1") {
            total += Math.pow(2, power);
          }
          power += 1;
        }
        return total;
      }
      var binaryNumber = "101010";
      try {
        var decimalNumber = binaryToDecimalManual(binaryNumber);
        document.getElementById("out").textContent =
          "Binary: " + binaryNumber + "\nDecimal: " + decimalNumber;
      } catch (err) {
        document.getElementById("out").textContent = err.message;
      }
    })();
  </script>
</body>
</html>

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JavaScript Binary to Decimal — Manual place value (Try It)