HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>.serialize() multi select</title>

  <style>

    body { font-family: sans-serif; padding: 16px; max-width: 560px; font-size: 14px; }

    select { width: 100%; height: 90px; }

    button { margin-top: 10px; padding: 8px 14px; cursor: pointer; background: #2563eb; color: #fff; border: none; border-radius: 6px; }

    .row { margin-top: 14px; display: grid; grid-template-columns: 1fr 1fr; gap: 10px; }

    .box { padding: 10px; background: #eff6ff; border-radius: 6px; font-size: 12px; word-break: break-all; }

    .label { font-weight: 600; color: #1e40af; margin-bottom: 6px; }

  </style>

</head>

<body>

  <form id="tags-form">

    <label>Tags (multi-select)

      <select name="tags" multiple>

        <option value="js" selected>JavaScript</option>

        <option value="jquery" selected>jQuery</option>

        <option value="ajax" selected>Ajax</option>

        <option value="css">CSS</option>

      </select>

    </label>

  </form>

  <button type="button" id="run">Compare selectors</button>

  <div class="row">

    <div class="box"><div class="label">$("#tags-form")</div><div id="correct">—</div></div>

    <div class="box"><div class="label">form + :input (wrong)</div><div id="wrong">—</div></div>

  </div>

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

  <script>

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

      $("#correct").text($("#tags-form").serialize());

      $("#wrong").text($("#tags-form, #tags-form :input").serialize());

    });

  </script>

</body>

</html>

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jQuery .serialize() — multiple select (Try It)