Node Express js Unhandled rejection Error [ERR_HTTP_HEADERS_SENT] - node.js

Question

I am new for node js. I am trying to insert the data to database, before that I did some validation for checking username already exist in database. Please see the coding which is used by me.
var express = require("express");
var router = express.Router();
var models = require("../models");
const { check, validationResult } = require('express-validator/check');
/* GET users listing. */
router.get('/', function(req, res, next) {
res.send('respond with a resource - merchant');
});
/* POST ADD USERS. */
router.post('/add-merchant', [
check('name').not().isEmpty(),
], (req, res, next) => {
try {
const errors = validationResult(req);
if (!errors.isEmpty()) {
return res.json({ errors: errors.array()});
}
console.log("error passed")
models.merchants.findOne({ where: {name: req.body.name}}).then((merchant) => {
if(merchant) {
throw new Error("Username already exists")
}
models.merchants.create(req.body).then((merchant) => {
res.json(merchant)
});
})
} catch (error) {
res.json({"status": "error", "message": error.message})
}
});
module.exports = router;
I am getting below error while running the above code
Unhandled rejection Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at ServerResponse.setHeader (_http_outgoing.js:470:11)
at ServerResponse.header (/home/assistanz/Desktop/node/gb360/node_modules/express/lib/response.js:767:10)
at ServerResponse.send (/home/assistanz/Desktop/node/gb360/node_modules/express/lib/response.js:170:12)
at ServerResponse.json (/home/assistanz/Desktop/node/gb360/node_modules/express/lib/response.js:267:15)
at models.merchants.create.then (/home/assistanz/Desktop/node/gb360/routes/merchant.js:33:21)
at tryCatcher (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/util.js:16:23)
at Promise._settlePromiseFromHandler (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/promise.js:512:31)
at Promise._settlePromise (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/promise.js:569:18)
at Promise._settlePromise0 (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/promise.js:614:10)
at Promise._settlePromises (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/promise.js:694:18)
at _drainQueueStep (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/async.js:138:12)
at _drainQueue (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/async.js:131:9)
at Async._drainQueues (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/async.js:147:5)
at Immediate.Async.drainQueues [as _onImmediate] (/home/assistanz/Desktop/node/gb360/node_modules/bluebird/js/release/async.js:17:14)
at runCallback (timers.js:705:18)
at tryOnImmediate (timers.js:676:5)
at processImmediate (timers.js:658:5)
Please help anyone to solve this issue.
Thanks in advance.

Answer 1

this error occurs because you return the response and something is remain to load
you have to remove console.log("error passed") or you can put it before return statement..
and write return statement for every res.json statement
as
return res.json({})

Answer 2

HTTP uses a cycle that requires one response per request. When the client sends a request the server should send only one response back to client. So modify the code as below,
models.merchants.findOne({ where: { name: req.body.name } }).then((merchant) => {
if (merchant) {
throw new Error("Username already exists")
} else {
models.merchants.create(req.body).then((merchant) => {
res.json(merchant)
});
}
})