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<!DOCTYPE html>
<html lang=en>
<head>
  <meta charset=utf-8>
  <title>Pairs from four numbers</title>
</head>
<body>
  <h1>All unordered pairs</h1>
  <pre id=out></pre>
  <script>!function(){var e,n,o=[10,20,30,40],t=[];for(t.push("All unordered pairs (choose 2 from 4):"),e=0;e<o.length;e++)for(n=e+1;n<o.length;n++)t.push(o[e]+" "+o[n]);document.getElementById("out").textContent=t.join("\n")}()</script>
</body>
</html>

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JavaScript Number Combination — Unordered Pairs (Try It)