Need help in correcting issues in CRC-ITU check method written in Javascript (node.js)

We are trying to code GPS device listener on Javascript. While doing this, we are unable to develop the right script for the CRC-ITU error check. The explanation for the crc code generation from protocol document is as below A check code may be used by the terminal or the server to distinguish whether the received information is error or not. To prevent errors occur during data transmission, error check is added to against data misoperation, so as to increase the security and efficiency of the system. The check code is generated by the CRC-ITU checking method. The check codes of data in the structure of the protocol, from the Packet Length to the Information Serial Number (including “Packet Length” and “Information Serial Number”) , are values of CRC-ITU. CRC error occur when the received information is calculated, the receiver will ignore and discard the data packet. We referred to the C code sample given in the protocol document - ``` // calculate the 16-bit CRC of data with predetermined length. U16 GetCrc16(const U8* pData, int nLength) { U16 fcs = 0xffff; // initialization while(nLength>0) { fcs = (fcs >> 8) ^ crctab16[(fcs ^ *pData) & 0xff]; nLength--; pData++; } return ~fcs; // negated } ``` the array crctab16 is the array same as mentioned in our code sample ``` Incoming data string comes in following format Example of data packet sent by the terminal 78 78 0D 01 01 23 45 67 89 01 23 45 00 01 8C DD 0D 0A Explanation Start Bit - 0x78 0x78 Length - 0x0D Protocol - 0x01 Terminal ID - 0x01 0x23 0x45 0x67 0x89 0x01 0x23 0x45 Serial No - 0x00 0x01 Error - 0x8C 0xDD Stop Bit - 0x0D 0x0A ``` Our Javascript code below ``` var net = require('net'); net.createServer(function(socket) { socket.setEncoding('hex'); // New client connection event socket.on('connect', function(data) // Incoming data event socket.on('data', function(data) { console.log("Client said: " + data); // server log data_p = data.substring(4,28); err_code = crc16(data_p); console.log("error: " + err_code); }); // Disconnect event socket.on('end', function() { //Log it to the server output console.log("someone left us." + socket.remoteAddress); }); }).listen(3006); console.log("TCP ECHO SERVER STARTED ON 3006"); function crc16(buf) { var crcTable = [ 0X0000, 0X1189, 0X2312, 0X329B, 0X4624, 0X57AD, 0X6536, 0X74BF, 0X8C48, 0X9DC1, 0XAF5A, 0XBED3, 0XCA6C, 0XDBE5, 0XE97E, 0XF8F7, 0X1081, 0X0108, 0X3393, 0X221A, 0X56A5, 0X472C, 0X75B7, 0X643E, 0X9CC9, 0X8D40, 0XBFDB, 0XAE52, 0XDAED, 0XCB64, 0XF9FF, 0XE876, 0X2102, 0X308B, 0X0210, 0X1399, 0X6726, 0X76AF, 0X4434, 0X55BD, 0XAD4A, 0XBCC3, 0X8E58, 0X9FD1, 0XEB6E, 0XFAE7, 0XC87C, 0XD9F5, 0X3183, 0X200A, 0X1291, 0X0318, 0X77A7, 0X662E, 0X54B5, 0X453C, 0XBDCB, 0XAC42, 0X9ED9, 0X8F50, 0XFBEF, 0XEA66, 0XD8FD, 0XC974, 0X4204, 0X538D, 0X6116, 0X709F, 0X0420, 0X15A9, 0X2732, 0X36BB, 0XCE4C, 0XDFC5, 0XED5E, 0XFCD7, 0X8868, 0X99E1, 0XAB7A, 0XBAF3, 0X5285, 0X430C, 0X7197, 0X601E, 0X14A1, 0X0528, 0X37B3, 0X263A, 0XDECD, 0XCF44, 0XFDDF, 0XEC56, 0X98E9, 0X8960, 0XBBFB, 0XAA72, 0X6306, 0X728F, 0X4014, 0X519D, 0X2522, 0X34AB, 0X0630, 0X17B9, 0XEF4E, 0XFEC7, 0XCC5C, 0XDDD5, 0XA96A, 0XB8E3, 0X8A78, 0X9BF1, 0X7387, 0X620E, 0X5095, 0X411C, 0X35A3, 0X242A, 0X16B1, 0X0738, 0XFFCF, 0XEE46, 0XDCDD, 0XCD54, 0XB9EB, 0XA862, 0X9AF9, 0X8B70, 0X8408, 0X9581, 0XA71A, 0XB693, 0XC22C, 0XD3A5, 0XE13E, 0XF0B7, 0X0840, 0X19C9, 0X2B52, 0X3ADB, 0X4E64, 0X5FED, 0X6D76, 0X7CFF, 0X9489, 0X8500, 0XB79B, 0XA612, 0XD2AD, 0XC324, 0XF1BF, 0XE036, 0X18C1, 0X0948, 0X3BD3, 0X2A5A, 0X5EE5, 0X4F6C, 0X7DF7, 0X6C7E, 0XA50A, 0XB483, 0X8618, 0X9791, 0XE32E, 0XF2A7, 0XC03C, 0XD1B5, 0X2942, 0X38CB, 0X0A50, 0X1BD9, 0X6F66, 0X7EEF, 0X4C74, 0X5DFD, 0XB58B, 0XA402, 0X9699, 0X8710, 0XF3AF, 0XE226, 0XD0BD, 0XC134, 0X39C3, 0X284A, 0X1AD1, 0X0B58, 0X7FE7, 0X6E6E, 0X5CF5, 0X4D7C, 0XC60C, 0XD785, 0XE51E, 0XF497, 0X8028, 0X91A1, 0XA33A, 0XB2B3, 0X4A44, 0X5BCD, 0X6956, 0X78DF, 0X0C60, 0X1DE9, 0X2F72, 0X3EFB, 0XD68D, 0XC704, 0XF59F, 0XE416, 0X90A9, 0X8120, 0XB3BB, 0XA232, 0X5AC5, 0X4B4C, 0X79D7, 0X685E, 0X1CE1, 0X0D68, 0X3FF3, 0X2E7A, 0XE70E, 0XF687, 0XC41C, 0XD595, 0XA12A, 0XB0A3, 0X8238, 0X93B1, 0X6B46, 0X7ACF, 0X4854, 0X59DD, 0X2D62, 0X3CEB, 0X0E70, 0X1FF9, 0XF78F, 0XE606, 0XD49D, 0XC514, 0XB1AB, 0XA022, 0X92B9, 0X8330, 0X7BC7, 0X6A4E, 0X58D5, 0X495C, 0X3DE3, 0X2C6A, 0X1EF1, 0X0F78 ]; crcX = parseInt("FFFF",16); cr1 = parseInt("FF",16); cr2 = parseInt("FFFF",16); i = 0; while(i < buf.length) { str = buf.substring(i,i+4); console.log("str "+str); str_hex = parseInt(str,16); j = (crcX ^ str_hex) & cr1; crcX = (crcX >> 8) ^ crcTable[j] ; i = i + 4; } crcX = ~crcX; console.log("crcX - " + crcX.toString(16)); return crcX; } ``` Please find below pairs of inputs strings and the corresponding code that should be generated by the function crc16 ``` 78780d01035889905017664600267bf90d0a - 7bf9 78780d010358899050176646002ab1950d0a - b195 78780d010358899050176646002ba01c0d0a - a01c ``` Please help us find the errors in our code. Much thanks.