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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>maxlength — 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}input{width:100%;max-width:320px;padding:.5rem .65rem;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}.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>element.maxLength in JavaScript</h1>
  <label for=dynamicField>Comment:</label>
  <input id=dynamicField maxlength=10 placeholder="Start with max 10">
  <button type=button id=extendBtn>Raise maxLength to 25</button>
  <p class=status id=status>Current maxLength: 10</p>

  <script>var field=document.getElementById("dynamicField"),status=document.getElementById("status");document.getElementById("extendBtn").addEventListener("click",(function(){field.maxLength=25,status.textContent="maxLength updated to 25 via element.maxLength property."}))</script>
</body>
</html>

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HTML maxlength Attribute — Dynamic maxLength with JavaScript