HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>relatedTarget — safe</title>
  <style>
    #card, #sidebar {
      display: inline-block;
      width: 120px;
      height: 80px;
      margin: 8px;
      border: 2px solid #333;
      text-align: center;
      line-height: 80px;
      user-select: none;
    }
  </style>
</head>
<body>
  <div id="card">card</div>
  <div id="sidebar">sidebar</div>
  <pre id="out">—</pre>
  <script>
    function relatedId(event) {
      return event.relatedTarget ? event.relatedTarget.id || "(no id)" : "unknown";
    }
    document.getElementById("card").addEventListener("mouseout", function (e) {
      document.getElementById("out").textContent =
        "left " + e.target.id + " toward " + relatedId(e);
    });
  </script>
</body>
</html>

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MouseEvent.relatedTarget — safe id helper (Try It)