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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 — inline handler</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}.note{padding:.75rem 1rem;background:#eff6ff;border:1px solid #bfdbfe;border-radius:8px;font-size:.8125rem;max-width:480px}code{background:#e2e8f0;padding:.1rem .35rem;border-radius:4px}</style>



</head>



<body onbeforeunload="return handleLeave()">



  <h1>onbeforeunload on body</h1>



  <p>Try closing this tab, reloading, or navigating away. The browser may show a generic leave confirmation.</p>



  <p class=note>Modern browsers ignore custom message text and show their own dialog. Returning a string (or using <code>preventDefault</code>) triggers the prompt.</p>





  <script>function handleLeave(){return"You may have unsaved changes."}</script>



</body>



</html>

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HTML onbeforeunload Attribute — Inline Handler on body