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<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Maximum value with index</title>
</head>
<body>
  <h1>Maximum and its index</h1>
  <p>Edit <code>array</code>, then <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      function findMaxWithIndex(arr) {
        if (!arr.length) {
          throw new Error("Array must contain at least one element.");
        }
        var maxVal = arr[0];
        var maxI = 0;
        var i;
        for (i = 1; i < arr.length; i++) {
          if (arr[i] > maxVal) {
            maxVal = arr[i];
            maxI = i;
          }
        }
        return { value: maxVal, index: maxI };
      }
      var array = [14, 7, 25, 31, 10, 42];
      var result = findMaxWithIndex(array);
      var lines = [];
      lines.push("Array: " + array.join(", "));
      lines.push("Maximum " + result.value + " found at index " + result.index);
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

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JavaScript Maximum Value — With Index (Try It)