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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>state() vs isRejected()</title>
</head>
<body>
  <h1>Modern state check</h1>
  <button id="ok">Resolve</button>
  <button id="bad">Reject</button>
  <pre id="out"></pre>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    function showState(dfd) {
      document.getElementById("out").textContent = [
        "state(): " + dfd.state(),
        "isRejected(): " + (dfd.state() === "rejected"),
        "state === rejected: " + (dfd.state() === "rejected")
      ].join("\n");
    }

    $("#ok").on("click", function () {
      var dfd = $.Deferred().resolve("done");
      showState(dfd);
    });

    $("#bad").on("click", function () {
      var dfd = $.Deferred().reject("failed");
      showState(dfd);
    });
  </script>
</body>
</html>

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jQuery deferred.isRejected() — state() comparison (Try It)