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<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

  <meta name=viewport content="width=device-width,initial-scale=1">

  <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 .75rem;font-size:1.25rem;color:#0f172a}label{display:block;margin-bottom:.35rem;font-weight:500;font-size:.875rem}select{width:100%;max-width:320px;padding:.5rem;font:inherit;border:1px solid #cbd5e1;border-radius:6px}button{margin-top:.75rem;padding:.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:.75rem;font-size:.8125rem;color:#64748b;max-width:420px}code{background:#e2e8f0;padding:.1rem .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"),status=document.getElementById("status");document.getElementById("enableBtn").addEventListener("click",(function(){sel.multiple=!0,sel.size=4,status.textContent="select.multiple: true — hold Ctrl/Cmd to pick several options."})),document.getElementById("disableBtn").addEventListener("click",(function(){sel.multiple=!1,sel.size=1,status.textContent="select.multiple: false (single selection)"}))</script>

</body>

</html>

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HTML multiple Attribute — Toggle multiple with JavaScript