HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>undefined does not overwrite</title>
  <style>
    body { font-family: sans-serif; padding: 16px; max-width: 28rem; }
    button { padding: 8px 16px; }
    #log { margin-top: 12px; padding: 12px; background: #f1f5f9; border-radius: 6px; }
  </style>
</head>
<body>
  <button type="button">Run handlers</button>
  <div id="log">Click the button.</div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("button").on("click", function () {
      return "first";
    });
    $("button").on("click", function () {
      // implicit undefined
    });
    $("button").on("click", function (event) {
      $("#log").text("result: " + event.result);
    });
  </script>
</body>
</html>

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jQuery event.result — undefined guard (Try It)