Unexpected result when my API server handels aws SNS subscriotion confirmation message - node.js

Question

I've created a topic in AWS SNS. Then I've created a subscription, in which an HTTP protocol is the endpoint .
I've written a simple script in express js to response to messages received from AWS-SNS.
The code expect to receive a json request. Then I'm trying to inspect the header of the request to decide on the appropriate response. If the header is SubscriptionConfirmation then the code will handle the request. And if the header is notification, it'll just print some message.
This is my code:
'use strict';
const express =require('express')
//const axios =require('axios')
//const got=require('got')
const request=require('request')
const app = express()
const bodyParser =require('body-parser')
const port = 4000
app.use(express.json())
// app.use(bodyParser.urlencoded({ extended: true }));
// app.use(bodyParser.json());
app.post('/', async function (req, res) {
try {
let valueStr = req.bod
let payload = JSON.parse(valueStr)
console.log(JSON.stringify(payload))
if (req.header('x-amz-sns-message-type') === 'SubscriptionConfirmation') {
const url = payload.SubscribeURL;
await request (url, handleSubscriptionResponse)
} else if (req.header('x-amz-sns-message-type') === 'Notification') {
console.log(payload)
//process data here
} else {
throw new Error(`Invalid message type ${payload.Type}`);
}
} catch (err) {
console.error(err)
res.status(500).send('Oops')
}
res.send('Ok')
})
var handleSubscriptionResponse = function (error, response) {
if (!error && response.statusCode == 200) {
console.log('Yess! We have accepted the confirmation from AWS');
}
else {
throw new Error(`Unable to subscribe to given URL`);
//console.error(error)
}
}
app.listen(port, () => {
console.log(`Example app listening at http://localhost:${port}`)
})
I've used ngrok to test my code. However, I've been getting some unexpected result when runing the script. The result shows some sort of errors which I'm not able to figure out:
Example app listening at http://localhost:4000
SyntaxError: Unexpected token u in JSON at position 0
at JSON.parse (<anonymous>)
at /home/ahmad/snstest/Sns.js:21:28
at Layer.handle [as handle_request] (/home/ahmad/snstest/node_modules/express/lib/router/layer.js:95:5)
at next (/home/ahmad/snstest/node_modules/express/lib/router/route.js:137:13)
at Route.dispatch (/home/ahmad/snstest/node_modules/express/lib/router/route.js:112:3)
at Layer.handle [as handle_request] (/home/ahmad/snstest/node_modules/express/lib/router/layer.js:95:5)
at /home/ahmad/snstest/node_modules/express/lib/router/index.js:281:22
at Function.process_params (/home/ahmad/snstest/node_modules/express/lib/router/index.js:335:12)
at next (/home/ahmad/snstest/node_modules/express/lib/router/index.js:275:10)
at jsonParser (/home/ahmad/snstest/node_modules/body-parser/lib/types/json.js:119:7)
(node:36762) UnhandledPromiseRejectionWarning: 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/ahmad/snstest/node_modules/express/lib/response.js:771:10)
at ServerResponse.send (/home/ahmad/snstest/node_modules/express/lib/response.js:170:12)
at /home/ahmad/snstest/Sns.js:36:9
at Layer.handle [as handle_request] (/home/ahmad/snstest/node_modules/express/lib/router/layer.js:95:5)
at next (/home/ahmad/snstest/node_modules/express/lib/router/route.js:137:13)
at Route.dispatch (/home/ahmad/snstest/node_modules/express/lib/router/route.js:112:3)
at Layer.handle [as handle_request] (/home/ahmad/snstest/node_modules/express/lib/router/layer.js:95:5)
at /home/ahmad/snstest/node_modules/express/lib/router/index.js:281:22
at Function.process_params (/home/ahmad/snstest/node_modules/express/lib/router/index.js:335:12)
(node:36762) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:36762) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
I'm a bit confused about the error !!3
I would appreciate any help on how to resolve this error.