HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>relatedTarget</title>
</head>
<body>
  <label>A <input id="a" type="text"></label>
  <label>B <input id="b" type="text"></label>
  <pre id="out">(tab between A and B)</pre>
  <script>
    var a = document.getElementById("a");
    var b = document.getElementById("b");
    var out = document.getElementById("out");
    function onFocus(event) {
      var from = event.relatedTarget
        ? (event.relatedTarget.id || event.relatedTarget.tagName)
        : "(none)";
      out.textContent =
        "focus on #" + event.target.id + " · relatedTarget=" + from;
    }
    a.addEventListener("focus", onFocus);
    b.addEventListener("focus", onFocus);
  </script>
</body>
</html>

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Element focus — relatedTarget (Try It)