HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>Toggle hidden class</title>

  <style>

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

    .hidden { display: none; }

  </style>

</head>

<body>

  <button type="button" id="showAll">Show hidden</button>

  <p class="hidden">Hidden paragraph 1</p>

  <p class="hidden">Hidden paragraph 2</p>

  <p>Visible paragraph</p>

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

  <script>

    $("#showAll").on("click", function() {

      $(".hidden").removeClass("hidden").fadeIn();

    });

  </script>

</body>

</html>

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Toggle .hidden elements on click (Try It)