HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>returnValue migrate</title>
</head>
<body>
  <a id="link" href="https://example.com" target="_blank" rel="noopener">Try modern cancel</a>
  <pre id="out">Waiting...</pre>
  <script>
    document.getElementById("link").addEventListener("click", function (event) {
      // Legacy (avoid): event.returnValue = false;
      if (event.cancelable) {
        event.preventDefault();
      }
      document.getElementById("out").textContent = event.defaultPrevented
        ? "Default was canceled the modern way"
        : "Could not cancel";
    });
  </script>
</body>
</html>

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Event.returnValue — migrate to preventDefault (Try It)