Minimal working version of call to AWS lambda to keep handy so I can screw up the one on my machine, er, I mean experiment with it.

lambda.js

const AWS = require("aws-sdk");

// Get soopur seekrit vars from .env file.
require("dotenv").config();  // could test response, but out of scope for simplicity
const REGION = process.env.REGION;
const IDENTITY_POOL_ID = process.env.IDENTITY_POOL_ID;
const FUNCTION_NAME = process.env.FUNCTION_NAME;

// Init cognito
AWS.config.region = REGION;
AWS.config.credentials = new AWS.CognitoIdentityCredentials({
  "IdentityPoolId": IDENTITY_POOL_ID
});

// Init params to pass to lambda function
const params = {
  "FunctionName" : FUNCTION_NAME,
  "InvocationType" : "RequestResponse",
  "LogType" : "None"
};

// Init lambda
const lambda = new AWS.Lambda({ "apiVersion": "2015-03-31" });

// Call the function and see what happens.
lambda.invoke(params, function(err, data) {
  if (err) {
    console.log(err);
  } else {
    console.log(JSON.parse(data.Payload).body);
  }
});