HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>document.cookie exists</title>
</head>
<body>
  <button id="check">Check cookie</button>
  <pre id="out"></pre>
  <script>
    document.cookie = "reader=1; path=/; SameSite=Lax";

    document.getElementById("check").onclick = function () {
      var hasReader = document.cookie
        .split(";")
        .some(function (item) {
          return item.trim().startsWith("reader=");
        });
      var isOne = document.cookie
        .split(";")
        .some(function (item) {
          return item.includes("reader=1");
        });
      document.getElementById("out").textContent =
        (hasReader
          ? '> The cookie "reader" exists\n'
          : '> The cookie "reader" does not exist\n') +
        (isOne ? '> Value is "1"' : '> Value is not "1"');
    };
  </script>
</body>
</html>

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Document.cookie — check exists (Try It)