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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>onbeforeunload — unsaved changes</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;font-weight:600;margin-bottom:.35rem}textarea{width:100%;max-width:420px;min-height:90px;padding:.5rem;font:inherit;border:1px solid #cbd5e1;border-radius:6px}.badge{display:inline-block;margin-top:.75rem;padding:.35rem .65rem;border-radius:6px;font-size:.75rem;font-weight:600;background:#dcfce7;color:#166534}.badge.dirty{background:#fee2e2;color:#991b1b}.hint{margin-top:.75rem;font-size:.8125rem;color:#64748b;max-width:420px}code{background:#e2e8f0;padding:.1rem .35rem;border-radius:4px}</style>



</head>



<body>



  <h1>Warn only when dirty</h1>



  <label for=draft>Draft note</label>



  <textarea id=draft placeholder="Type something, then try to leave…"></textarea>



  <span class=badge id=badge>Saved — no leave warning</span>



  <p class=hint>Best practice: attach <code>beforeunload</code> only after the user changes something, or remove the listener when they save.</p>





  <script>var dirty=!1,draft=document.getElementById("draft"),badge=document.getElementById("badge");draft.addEventListener("input",(function(){dirty=draft.value.trim().length>0,badge.textContent=dirty?"Unsaved — leave warning active":"Saved — no leave warning",badge.classList.toggle("dirty",dirty)})),window.addEventListener("beforeunload",(function(e){dirty&&(e.preventDefault(),e.returnValue="")}))</script>

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HTML onbeforeunload Attribute — Unsaved Changes Only