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<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>has registered handler</title>

</head>

<body>

  <h1>Registered vs unregistered</h1>

  <pre id="out"></pre>

  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>

  <script>

    (function () {

      var lines = [];

      var callbacks = $.Callbacks();



      function onReady() { console.log("Ready!"); }

      function onError() {}



      callbacks.add(onReady);

      lines.push("has(onReady): " + callbacks.has(onReady));

      lines.push("has(onError): " + callbacks.has(onError));



      document.getElementById("out").textContent = lines.join("\n");

    })();

  </script>

</body>

</html>

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jQuery callbacks.has() — registered vs not (Try It)