How do i return a value from a node.js function using a callback?

Based on the above help from Sagar, I've now been able to get my head around callbacks, promises and await in nodejs. (Well I think so anyway.) The code snippet below is now a working example extracted from my Lambda function. The rest of the code is just based on samples available on Github or AWS.

My solution was to use a callback for one part and async/promise/await for the code in the Alexa handler code. This probably could now be reduced to something simpler so the code is not repeated in each handler, but for what it is worth, I hope this code below helps some other newbie to nodejs and AWS IOT/Alexa to work out how to get the Lamda to wait to get the values needed from the IOT thing shadow.

// more code above ...(see ASK and AWS samples)

//IOT Stuff - settings
AWS.config.region = "<put your own region in here>";
var IOT_Data = new AWS.IotData({
    endpoint: "<put your IOT Shadow Endpoint in here>"
});
var IOT_Thing = {
    "thingName": "<put the name of your thing here>"
};
var doorstate; //this is used to hold the part of JSON data I want 

//additional functions
//get shadow of thing using a call back.  this function uses a nodejs callback 
function getdoorstate(callback) {
    IOT_Data.getThingShadow(IOT_Thing, function(err, data) {
        if (err) {
            console.log("Error : " + err, err.stack);
        } else {
            callback(data);
        }
    });
}

//..more code in here.. not shown in this snippet

//this is one of the Alexa intent handlers

// core functionality
const DoorStateHandler = {
    canHandle(handlerInput) {
        const request = handlerInput.requestEnvelope.request;
        // checks request type - this one handes an intent with name of doorState
        return request.type === 'IntentRequest' &&
            request.intent.name === 'doorState';
    },
// important to use the async here to get the code to wait for the 'promise' (uses nodejs promise and await)
    async handle(handlerInput) {
        var speakOutput = "Let me check..."; /set a var ready for the speech to send back to Alexa
        //set up a promise so that we can get the value I need from the thing shadow
    let promise = new Promise((resolve, reject) => {
            getdoorstate(function(value) {
                var obj = JSON.parse(value.payload); //parse to json
                doorstate = obj["state"]["reported"]["doorstate"]; //return just the value of the doorstate
                console.log("The doorstate is: " + doorstate);
                speakOutput = speakOutput.concat(msg[doorstate]); //concatenate this to the var containing the speech
                resolve(speakOutput); //resolve the promise and return the speech
            });
        });
        //return result
    // this is important - use the await here so that we dont proceed any further until the promise is resolved with the thing values
        let result = await promise; 
        return handlerInput.responseBuilder
            .speak(result)
            .withSimpleCard("Garage Door", result)
            .getResponse();
    },
};

// this is another handler for sending a desired action to the IOT shadow 
const DoorTransitHandler = {
    canHandle(handlerInput) {
        const request = handlerInput.requestEnvelope.request;
        // checks request type
        return request.type === 'IntentRequest' &&
            request.intent.name === 'activate_door';
    },
    //again use async so that we can use promise and await - all the rest like above..
    async handle(handlerInput) {
        var speakOutput = "I'm on it...";
        let promise = new Promise((resolve, reject) => {
            getdoorstate(function(value) {
                var obj = JSON.parse(value.payload);
                doorstate = obj["state"]["reported"]["doorstate"];
                console.log("The doorstate is: " + doorstate);
                speakOutput = speakOutput.concat(msg[doorstate]);
                resolve(speakOutput);
            });
        });
        //return result
        let result = await promise;
        // set output
        result += "I'll push the button for you. ";

    //build the json to send back to the IOT shadow to get the thing to do something
        var paramsUpdate = {
            "topic": "$aws/things/garagedoor1/shadow/update",
            "payload": '{"state":{"desired":{"doorstate":0,"transitstate":1}}}',
            "qos": 0
        };
    //now here we send our desired action back to the IOT Thing Shadow
        IOT_Data.publish(paramsUpdate, function(err, data) {
            if (err) console.log(err, err.stack); // an error occurred
            else console.log(data.payload); // successful response
        });

        // return response by means of Alexa
        return handlerInput.responseBuilder
            .speak(result)
            .getResponse();
    },
};

// and the code continues...  (see ASK and AWS samples)