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  <title>Factorial — BigInt</title>
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  <h1>Exact factorial with BigInt</h1>
  <p>Edit values in <code>tests</code>, then click <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      function factorialBigInt(n) {
        if (n < 0) {
          throw new Error("Factorial is not defined for negative integers.");
        }
        var result = 1n;
        var i;
        for (i = 2n; i <= BigInt(n); i++) {
          result *= i;
        }
        return result;
      }
      var tests = [0, 1, 5, 10, 20];
      var lines = [];
      for (var j = 0; j < tests.length; j++) {
        var n = tests[j];
        lines.push(n + "! = " + factorialBigInt(n));
      }
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
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JavaScript Factorial — BigInt exact (Try It)