HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>jQuery only-child demo</title>
  <style>
    body { font-family: sans-serif; padding: 16px; }
   div { width: 120px; height: 80px; margin: 5px; float: left; background: #b9e; padding: 8px; box-sizing: border-box; }
    button { margin: 4px 0; }
  </style>
</head>
<body>
  <div><button>Btn</button> Sibling! <button>Btn</button></div>
  <div><button>Btn</button></div>
  <div>Sibling! <button>Btn</button> Sibling!</div>
  <div><button>Btn</button><button>Btn</button></div>
  <div><button>Btn</button></div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("div button:only-child").text("Alone").css("border", "2px blue solid");
  </script>
</body>
</html>

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jQuery div button:only-child — official demo (Try It)