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<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Number Pattern 51 - Try It (Browser)</title>
</head>
<body>
  <h1>Alternating Row Number Triangle (aligned cells)</h1>
  <p>Edit rows and click Run.</p>

  <script>
    var rows = 5;
    var k = 1;

    for (var i = 1; i <= rows; i++) {
      var start = k;
      var end = k + i - 1;

      if (i % 2 === 1) {
        for (var v = start; v <= end; v++)
          document.write('<span style="display:inline-block;width:2.5em;text-align:right">' + v + '</span>');
      } else {
        for (var v2 = end; v2 >= start; v2--)
          document.write('<span style="display:inline-block;width:2.5em;text-align:right">' + v2 + '</span>');
      }

      document.write("<br>");
      k = end + 1;
    }
  </script>
</body>
</html>

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JavaScript Number Pattern 51 (Try It — Browser)