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public static string decompressText(string compressedText)
       {
           compressedText = compressedText.Replace("uFEFF", "");
           //compressedText = "CQAAAB+LCAAAAAAABABzzs/LS00uSU0BABaaRpgJAAAA";
           byte[] gzBuffer = Convert.FromBase64String(compressedText);
           using (MemoryStream ms = new MemoryStream())
           {
               int msgLength = BitConverter.ToInt32(gzBuffer, 0);
               ms.Write(gzBuffer, 4, gzBuffer.Length - 4);

               byte[] buffer = new byte[msgLength];
               ms.Position = 0;
               using (GZipStream zip = new GZipStream(ms, CompressionMode.Decompress))
               {
                   zip.Read(buffer, 0, buffer.Length);
               }
               return Encoding.UTF8.GetString(buffer);
           }
       }

<!DOCTYPE html>
<head>
    <script src="path_to/some_compression_decompression.js"></script>
    <script>
    var text = 'compressedData';
    var libObj = new some_compression_decompression_class();
    console.log(libObj.decompress(text));       // output decompressed data
    </script>
<head>
<body>
</body>
</html>