HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>first-child vs first</title>

  <style>

    body { font-family: sans-serif; padding: 16px; }

    li { margin: 4px 0; padding: 4px 8px; border: 1px solid #ccc; }

    .fc { background: #bbf; }

    .one { outline: 3px solid #dc2626; }

    #output { margin-top: 12px; padding: 12px; background: #e0f2fe; border: 1px solid #0284c7; }

  </style>

</head>

<body>

  <ul><li>A1</li><li>A2</li></ul>

  <ul><li>B1</li><li>B2</li></ul>

  <div id="output"></div>

  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>

  <script>

    $("li:first-child").addClass("fc");

    $("li").first().addClass("one");

    $("#output").html(

      ":first-child → " + $("li:first-child").length + " (one per list)<br>" +

      ".first() → " + $("li").first().length + " (single element)"

    );

  </script>

</body>

</html>

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:first-child vs .first() (Try It)