Cant set headers after they are sent - node.js

Question

I am using express backend with a react frontend everything is working fine but occasionally i get error
Cant set header after they are sent
and server gets down.i searched few ways this error might happen but in my code i could not find such cases.i tried to be simple as possible in the code.can anyone please point me what might be the issue?
Server.js file
// call the packages we need
const addItem = require('./controllers/addItem');
const addCategory = require('./controllers/addCategory');
const addSubCategory = require('./controllers/addSubCategory');
const getSubCategory = require('./controllers/getSubCategoryByCategory');
const getCategory = require('./controllers/getAllCategory');
const getAllItems = require('./controllers/getAllItems');
const cors = require('cors');
const express = require('express');
// call express
const app = express(); // define our app using express
const bodyParser = require('body-parser');
// configure app to use bodyParser()
// this will let us get the data from a POST
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
app.use(cors());
const port = process.env.PORT || 8080; // set our port
// ROUTES FOR OUR API
// =============================================================================
const addItemRoute = express.Router(); // get an instance of the express Router
const getCategoryRoute = express.Router();
const addCategoryRoute = express.Router();
const addSubCategoryRoute = express.Router();
const getSubCategoryRoute = express.Router();
const getAllItemsRoute = express.Router();
getCategoryRoute.get('/get_category', (req, res) => {
getCategory(res);
});
addCategoryRoute.post('/add_category', (req, res) => {
addCategory(req.body.name, res);
});
getSubCategoryRoute.get('/get_subcategory/:catId', (req, res) => {
getSubCategory(req.params.catId, res);
});
addSubCategoryRoute.post('/add_subcategory', (req, res) => {
addSubCategory(req.body.name, req.body.cat_id, res);
});
// code, name, quantity, length, description and subcategory id should be passed as parameters
addItemRoute.post('/add_item', (req, res) => {
addItem(req.body.item, res);
});
getAllItemsRoute.get('/get_items', (req, res) => {
getAllItems(res);
});
// more routes for our API will happen here
// REGISTER OUR ROUTES -------------------------------
// all of our routes will be prefixed with /api
app.use('/api', addItemRoute);
app.use('/api', getCategoryRoute);
app.use('/api', addCategoryRoute);
app.use('/api', addSubCategoryRoute);
app.use('/api', getSubCategoryRoute);
app.use('/api', getAllItemsRoute);
// START THE SERVER
// =============================================================================
app.listen(port);
console.log(`Server started on port ${port}`);
getAllCategories() function
Object.defineProperty(exports, '__esModule', {
value: true,
});
const pool = require('./connection');
module.exports = function (res) {
pool.getConnection((err, connection) => {
if (err) {
connection.release();
return res.json({ code: 100, status: 'Error in connection database' });
}
console.log(`connected as id ${connection.threadId}`);
connection.query('select * from category;', (err, rows) => {
connection.release();
if (!err) {
return res.json(rows);
}
});
connection.on('error', err => res.json({ code: 100, status: 'Error in connection database' }));
});
};

Answer 1

If you get an error in connection.query() you send a response with res.json(). This error is caught in connection.on('error') where you send another response. You can't send two responses to the same request. It seems that in this case, you don't really need connection.on() at all or if you have it to catch other errors, don't send a response on connection.query()'s error.