HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>relatedTarget — null</title>
</head>
<body>
  <button id="btn" type="button">Click me</button>
  <pre id="out">—</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function (e) {
      document.getElementById("out").textContent = [
        String(e.relatedTarget),
        e.target.tagName
      ].join("\n");
    });
  </script>
</body>
</html>

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MouseEvent.relatedTarget — null on click (Try It)