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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>JSON comments — invalid</title>
</head>
<body>
  <h1>JSON.parse + // comment</h1>
  <pre id="out"></pre>
  <script>
    (function () {
      // This text is invalid JSON because of the // comment
      var text = `{
  // not allowed in JSON
  "name": "Ada"
}`;

      var out = document.getElementById("out");
      try {
        JSON.parse(text);
        out.textContent = "Parsed OK (unexpected)";
      } catch (err) {
        out.textContent = err.name + ": " + err.message;
      }
    })();
  </script>
</body>
</html>

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JSON Comments — // fails (Try It)