Why does AWS Lambda function return null? - node.js

Question

My AWS Lambda function sends an email through SNS, but the response is null in the Lambda console. I used a blueprint provided by AWS and put the code in a Lambda handler using Node.js 10.x.
https://github.com/awsdocs/aws-doc-sdk-examples/blob/master/javascript/example_code/sns/sns_publishtotopic.js
I am not that experienced in the use of promises in node.js and my research on Stack Overflow seems to indicate that could be the problem here.
'use strict';
var AWS = require('aws-sdk');
// Set region
AWS.config.update({region: 'us-west-2'});
exports.handler = (event, context, callback) => {
// Create publish parameters
var params = {
//Message: `The fluid level is low on your Knight system: ${event.my_volume} mL.`,
Message: `The fluid level is low on your Knight system.`,
Subject: `Low Fluid Level Alert - Knight`,
TopicArn: `arn:aws:sns:us-west-2:468820349153:MyLowVolumeTopic`,
};
// Create promise and SNS service object
var publishTextPromise = new AWS.SNS({apiVersion: '2010-03-31'}).publish(params).promise();
// Handle promise's fulfilled/rejected states
publishTextPromise.then(
function(data) {
console.log("Message: " + params.Message);
console.log("Sent to the topic: " + params.TopicArn);
console.log("MessageID is " + data.MessageId);
}).catch(
function(err) {
console.error(err, err.stack);
});
};
The result is that I receive the email message inconsistently and see a null response in the Lambda console. Here is a sample log result:
Response: null
Request ID: "9dcc8395-5e17-413a-afad-a86b9e04fb97"
Function Logs:
START RequestId: 9dcc8395-5e17-413a-afad-a86b9e04fb97 Version: $LATEST
2019-08-17T21:44:31.136Z 9dcc8395-5e17-413a-afad-a86b9e04fb97 Message: The fluid level is low on your Knight system.
2019-08-17T21:44:31.136Z 9dcc8395-5e17-413a-afad-a86b9e04fb97 Sent to the topic: arn:aws:sns:us-west-2:468820349153:MyLowVolumeTopic
2019-08-17T21:44:31.136Z 9dcc8395-5e17-413a-afad-a86b9e04fb97 MessageID is cb139fb6-5d37-574e-9134-ca642a49fde5
END RequestId: 9dcc8395-5e17-413a-afad-a86b9e04fb97
REPORT RequestId: 9dcc8395-5e17-413a-afad-a86b9e04fb97 Duration: 1367.98 ms Billed Duration: 1400 ms Memory Size: 128 MB Max Memory Used: 75 MB

Answer 1

Your lambda is exiting before completing the promise. To circumvent this you can use async-await:
exports.handler = async (event, context, callback) => {
const data = await publishTextPromise;
//maybe some console log here
return data;
}
OR, You can return the promise, return publishTextPromise.then(...).catch(...)