How to send json data to client from node js server?

Question

I have a device that says following points its api documentation:

1. Use websocket protocol to communication,the websocket version is RFC6455 13,The default listen port is 7788,no TLS encrypt.
2. The data format use Json.you can use javascript to Serializer and Deserialize very easy. 
3. All the key value of json use lower-char.the name or all chinese char use UTF8 encoded.

I am creating nodejs app as a server to this device.

var net = require("net");

var server = net.createServer();

server.on("connection",function(socket){
    socket.on("data",function(d){
       console.log("Data from  %s : %s", remoteAddress,d);
     socket.write(
        {
        "cmd":"getuserlist",
        "stn":true
        }
    );
    });
});
server.listen(9000,function(){
    console.log("Server listening to port %j", server.address());
});

The device is being connected. After connection i am sending json data to the device and according to the documentation it also needs to respond json data but it is not working. Please help me to get the response. Further i neeed to call to an url by parsing the json response. let me make clear.

//If i get response like:
success:{
  "count":40,
}
//I need to call an url like:
"http://example.com/40/"

How can i achieve it ?
P.S: I am new to node.js

remoteAddress is not declared, socket.write accepts only string-s (try JSON.stringify({...})) — Ihor Sakailiuk
– Ihor Sakailiuk, Commented Nov 24, 2019 at 12:47

Ihor Sakailiuk · Accepted Answer · 2019-11-24 13:44:39Z

3

There are couple of issues with the code provided above:

remoteAddress never declared, but used in the log statement
socket.write accepts only string arguments, so you need to stringify JS object before sending it
You have specified that you need to send JSON-data to the client (device) after the connection, but you're actually sending it as a response to the client's requests.
You've specified "The default listen port is 7788" but your server is listening on port 9000
And finally, you've specified that the communication protocol is websocket-s not a tcp-sockets.

Example with TCP-sockets:

const net = require("net");
const server = net.createServer();

server.on("connection", function(socket) {
    const remoteAddress = socket.remoteAddress;

    socket.write(JSON.stringify({
        cmd: "getuserlist",
        stn: true
    }));

    socket.on("data", function(d) {
        const data = JSON.parse(d);
        console.log("Data from  %s : %j", remoteAddress, data);

        if (data.success && data.success.count == 40) {
          // TODO: Call an url ...
        }
    });
});

server.listen(7788, function() {
    console.log("Server listening to port %j", server.address());
});

You can test the tcp-server using, for example telnet

$ telnet localhost 7788

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
{"cmd":"getuserlist","stn":true}
{"success": {"count": 40}}

Example with Web-sockets:

npm i -S ws

const WebSocket = require('ws');
const wss = new WebSocket.Server({ port: 7788 });

wss.on('connection', function (ws, req) {
    const remoteAddress = req.connection.remoteAddress
    ws.send(JSON.stringify({
        cmd: "getuserlist",
        stn: true,
    }));

    ws.on('message', function (d) {
        const data = JSON.parse(d);
        console.log("Data from  %s : %j", remoteAddress, data);

        if (data.success && data.success.count == 40) {
            // TODO: Call an url ...
        }
    });
});

You can test web-socket server using wscat (or telnet as well).

npm i -g wscat

$ wscat -c ws://localhost:7788

Connected (press CTRL+C to quit)
< {"cmd":"getuserlist","stn":true}
> {"sucess": {"count": 40}}

edited Nov 24, 2019 at 13:44

answered Nov 24, 2019 at 13:04

Ihor Sakailiuk

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4 Comments

Bishal Chhetri Over a year ago

Thankyou. Bt i am still getting undefined:1�Z^ as "data" .Unexpected token � in JSON at position 0 at JSON.parse

Ihor Sakailiuk Over a year ago

So, the client not sending a request with JSON format to the server. Try to debug it: const data = Buffer.from(d, 'utf-8'); and console.log("Data from %s : %s", remoteAddress, data);

Bishal Chhetri Over a year ago

It still says Data from ::ffff:192.168.1.224 : {"type":"Buffer","data":[165,90,1,0,154,69,220,33,1,0,0,0,0,0,221,2]} events.js:174 throw er; // Unhandled 'error' event ^ Error: read ECONNRESET at TCP.onStreamRead (internal/stream_base_commons.js:111:27)

Bishal Chhetri Over a year ago

Thankyou soo much @lhor Sakaylyuk

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