https://www.losant.com/blog/aws-lambda-and-losant-workflows

user-lookup.js

'use strict';
const doc = require('dynamodb-doc');
const dynamo = new doc.DynamoDB();

exports.handler = (event, context, callback) => {
    if(!event || !Array.isArray(event)){ return callback(null, null); }
    
    let deviceSerial;
    event.forEach((pair) => {
        if(pair.key === 'deviceSerial'){ deviceSerial = pair.value; }
    });
    if(!deviceSerial){ return callback(null, null); }

    let deviceQuery = { TableName: 'deviceMapping', Key: { deviceSerial } };
    dynamo.getItem(deviceQuery, (err, result) => {
        if(err){ return callback(err); }
        if(!result.Item){ return callback(null, null); }
        
        let userQuery = { TableName: 'users', Key: { id: result.Item.userId } };
        dynamo.getItem(userQuery, (err, result) => {
            if(err){ return callback(err); }
            if(!result.Item){ return callback(null, null); }
            callback(null, result.Item);
        });
    });
};