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  <title>onbeforeunload — addEventListener</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}.status{margin-top:1rem;padding:.75rem 1rem;background:#fef9c3;border:1px solid #fde047;border-radius:8px;font-size:.8125rem;max-width:480px}code{background:#e2e8f0;padding:.1rem .35rem;border-radius:4px}</style>



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  <h1>window.addEventListener("beforeunload")</h1>



  <p>Preferred modern pattern — call <code>event.preventDefault()</code> to show the browser&rsquo;s leave dialog.</p>



  <p class=status id=status>Leave warning is active. Try closing this tab or reloading.</p>





  <script>window.addEventListener("beforeunload",(function(e){e.preventDefault(),e.returnValue=""}))</script>



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HTML onbeforeunload Attribute — addEventListener