HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Decode cookie value</title>
</head>
<body>
  <h1>Decode value</h1>
  <button type="button" id="run">Set and read greeting</button>
  <pre id="out"></pre>
  <script>
    function getCookie(name) {
      var pairs = document.cookie.split(";");
      for (var i = 0; i < pairs.length; i++) {
        var trimmed = pairs[i].trim();
        var eq = trimmed.indexOf("=");
        if (eq === -1) continue;
        if (trimmed.slice(0, eq) === name) {
          try {
            return decodeURIComponent(trimmed.slice(eq + 1));
          } catch (e) {
            return trimmed.slice(eq + 1);
          }
        }
      }
      return null;
    }

    document.getElementById("run").addEventListener("click", function () {
      document.cookie = "greeting=" + encodeURIComponent("Hello, world!") + "; path=/";
      document.getElementById("out").textContent = getCookie("greeting");
    });
  </script>
</body>
</html>

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JavaScript Cookies Read — decodeURIComponent (Try It)