Invalid JSON when calling Node 8.10 Lambda - node.js

Question

I am using lambda with cognito to write to dynamoDB after a successful login.
Node 8.10 has a different layout with the promise and asycn/await. the callback(null, event) return is not working for my. Anyone now how to solve the problem of Invalid lambda function output : Invalid JSON with node 8.10.
// Load the AWS SDK for Node.js
var AWS = require('aws-sdk');
// Set the region
//AWS.config.update({region: 'REGION'});
// Create DynamoDB document client
var docClient = new AWS.DynamoDB.DocumentClient({apiVersion: '2012-08-10'});
exports.myHandler = async (event, context, callback) => {
// TODO implement
console.log ("Authentication successful");
console.log ("Trigger function =", event.triggerSource);
console.log ("User pool = ", event.userPoolId);
console.log ("App client ID = ", event.callerContext.clientId);
console.log ("User ID = ", event.userName);
const params = {
TableName: 'xxxx',
Item: {
'userId': event.userName,
'systemUpdateDate': new Date().toJSON()
}
};
let putItem = new Promise((res, rej) => {
docClient.put(params, function(err, data) {
if (err) {
console.log("Error", err);
} else {
console.log("Success", data);
}
});
});
const result = await putItem;
console.log(result);
// Return to Amazon Cognito
callback(null, event);
};
thanks

Answer 1

With the suggested Node 8 approach of async/await you should use the following approach to structuring your function:
async function handler(event) {
const response = doSomethingAndReturnAJavascriptObject();
return response;
}
You're getting that error because whatever you're returning isn't something that can be parsed as a JSON object.
Without seeing your code it's hard to debug further. I expect that you might accidentally not be awaiting a .promise() version of a dynamo/cognito API call, which is causing you to return a Promise instead of a result.
n.b. you can still use the 'old' callback() method with Node 8 if you find it easier.