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<head>
  <meta charset="utf-8">
  <title>jQuery.parseJSON() — invalid</title>
</head>
<body>
  <pre id="out"></pre>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    (function () {
      var badSamples = ['{test: 1}', "{'test': 1}", '"undefined"', '".1"'];
      var lines = [];
      badSamples.forEach(function (sample) {
        try {
          $.parseJSON(sample);
          lines.push("Unexpected success: " + sample);
        } catch (err) {
          lines.push("Rejected: " + sample.slice(0, 14));
        }
      });
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

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jQuery.parseJSON() — invalid JSON (Try It)