HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>UI error with fail()</title>
  <style>
    #msg { margin-top: 0.75rem; color: #b91c1c; font-weight: 600; }
  </style>
</head>
<body>
  <h1>Failed request handler</h1>
  <button id="fetch">Fetch items</button>
  <div id="msg"></div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    function fetchItems() {
      var dfd = $.Deferred();
      $("#msg").text("");

      setTimeout(function () {
        dfd.reject("503 Service Unavailable");
      }, 600);

      return dfd
        .done(function (items) {
          $("#msg").css("color", "#0f766e").text("Loaded: " + items.join(", "));
        })
        .fail(function (error) {
          $("#msg").text("Could not load items — " + error);
        });
    }

    $("#fetch").on("click", fetchItems);
  </script>
</body>
</html>

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jQuery deferred.fail() — user-facing error message (Try It)