HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>firstElementChild children</title>
</head>
<body>
  <div id="box">
    <p>First</p>
    <p>Second</p>
  </div>
  <pre id="out"></pre>
  <script>
    var box = document.getElementById("box");
    var lines = [];
    lines.push(String(box.firstElementChild === box.children[0]));
    lines.push(box.firstElementChild.tagName);
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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Element.firstElementChild — equals children[0] (Try It)