HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>presence vs exact</title>

  <style>

    body { font-family: sans-serif; padding: 16px; max-width: 28rem; }

    div { padding: 8px; margin: 6px 0; border: 1px solid #e2e8f0; border-radius: 6px; }

    #output { margin-top: 12px; padding: 12px; background: #fef3c7; font-family: monospace; white-space: pre-wrap; }

  </style>

</head>

<body>

  <div id="foo">id = foo</div>

  <div id="bar">id = bar</div>

  <div>no id</div>

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

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

  <script>

    $("#output").text(

      "Has id → " + $("div[id]").length + "\nid=\"foo\" → " + $('div[id="foo"]').length

    );

    $('div[id="foo"]').css("background", "#bbf");

  </script>

</body>

</html>

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[id] vs [id="foo"] comparison (Try It)