HTML Code
<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="UTF-8">

  <meta name="viewport" content="width=device-width, initial-scale=1.0">

  <title>multiple — dynamic JS</title>

  <style>

    body {

      font-family: system-ui, sans-serif;

      margin: 0;

      padding: 1.5rem;

      background: #f8fafc;

      color: #334155;

      line-height: 1.7;

    }

    h1 { margin: 0 0 0.75rem; font-size: 1.25rem; color: #0f172a; }

    label { display: block; margin-bottom: 0.35rem; font-weight: 500; font-size: 0.875rem; }

    select {

      width: 100%;

      max-width: 320px;

      padding: 0.5rem;

      font: inherit;

      border: 1px solid #cbd5e1;

      border-radius: 6px;

    }

    button {

      margin-top: 0.75rem;

      padding: 0.5rem 1rem;

      font: inherit;

      cursor: pointer;

      border-radius: 6px;

      border: 1px solid #cbd5e1;

      background: #fff;

    }

    button.is-on {

      background: #2563eb;

      color: #fff;

      border-color: #1d4ed8;

    }

    .status {

      margin-top: 0.75rem;

      font-size: 0.8125rem;

      color: #64748b;

      max-width: 420px;

    }

    code { background: #e2e8f0; padding: 0.1rem 0.35rem; border-radius: 4px; }

  </style>

</head>

<body>

  <h1>select.multiple in JavaScript</h1>

  <label for="colors">Pick a color:</label>

  <select id="colors" name="colors" size="4">

    <option value="red">Red</option>

    <option value="green">Green</option>

    <option value="blue">Blue</option>

    <option value="yellow">Yellow</option>

  </select>

  <div>

    <button type="button" id="enableBtn">Enable multiple</button>

    <button type="button" id="disableBtn">Disable multiple</button>

  </div>

  <p class="status" id="status">select.multiple: false (single selection)</p>



  <script>

    var sel = document.getElementById("colors");

    var status = document.getElementById("status");

    document.getElementById("enableBtn").addEventListener("click", function () {

      sel.multiple = true;

      sel.size = 4;

      status.textContent = "select.multiple: true — hold Ctrl/Cmd to pick several options.";

    });

    document.getElementById("disableBtn").addEventListener("click", function () {

      sel.multiple = false;

      sel.size = 1;

      status.textContent = "select.multiple: false (single selection)";

    });

  </script>

</body>

</html>
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HTML multiple Attribute — Toggle multiple with JavaScript
