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<head>
  <meta charset="utf-8">
  <title>Composite — witness factor</title>
</head>
<body>
  <h1>Find a witness factor</h1>
  <p>Edit the values in <code>samples</code>, then click <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      function firstFactor(number) {
        if (number <= 1) {
          return null;
        }
        var i = 2;
        while (i * i <= number) {
          if (number % i === 0) {
            return i;
          }
          i += 1;
        }
        return null;
      }
      var samples = [12, 7, 1, 9];
      var lines = [];
      for (var idx = 0; idx < samples.length; idx++) {
        var n = samples[idx];
        var f = firstFactor(n);
        if (f === null) {
          var label = n <= 1 ? "neither" : "prime";
          lines.push(n + ": not composite (" + label + ")");
        } else {
          lines.push(n + ": composite (divisible by " + f + ")");
        }
      }
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

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JavaScript Composite Number — witness factor (Try It)