Read Serial port data using node js

Question

I want to read data from serial port and get from data when reqested

Here is my code

const http = require('http');
const hostname = 'localhost';
const { SerialPort } = require('serialport')
const { ReadlineParser } = require('@serialport/parser-readline')
const { io } = require('socket.io');
let express = require('express')

const serialPort = new SerialPort({ 
    path: 'COM4',
    baudRate: 9600 ,
})
const parser = serialPort.pipe(new ReadlineParser({ delimiter: '\r\n' }))
let app = express();
var port = 8080;

const server = http.createServer(app);

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});
app.get('/get_data', function(req, res) {
    parser.on('data', function(data) {
        res.json({'weight': data});
    });
});

When i am try to get data i got ERR_HTTP_HEADERS_SENT: Cannot set headers after they are sent to the client I want serial port data when requested from localhost:8080/get_data anyone can help?

Answer 1

Your data event from parser is probably firing more than once, which means you would be calling res.json more than once. As you can see in the express api documentation , res.json sets the content-type header...thus you can only call it once per request. Hence the error.

What I think would normally be done in this kind of situation is to set up a queuing system. A simple version might be done using an array, although if you were using this in a production server it might be better to use a proper message queuing system (eg rabbitMQ, kafka, AWS SQS, etc).

Here's an example of how you might use an array:

const queue = [];
parser.on('data', function(data) {
  // push new data onto end of queue (array)
  queue.push(data);
});
app.get('/get_data', function(req, res) {
  if (req.params.getFullQueue === 1) {
    // empty complete contents of current queue,
    // sent to client as an array of { weight: x } objects
    const data = queue.splice(0, queue.length)
      .map(x => ({ weight: x }));
    res.json(data);
  } else {
    // get oldest enqueued item, send it only
    res.json({ weight: queue.shift() });
  }
});

The if/else in the app.get is meant to illustrate these two options, depending on which you wanted to use. In production you'd probably want to implement pagination, or maybe even a websocket or EventSource so that data could be pushed as it became available.