How to return json from callback function within the Lambda?

Question

I'm trying to return the login status from the Cognito callback function, which is written in the NodeJS Lambda. However when I call the API the response keep loading and I'm getting warning error.

Here is my code:

'use strict';

global.fetch = require('node-fetch');
const AmazonCognitoIdentity = require('amazon-cognito-identity-js');

module.exports.hello = async (event, context) => {
  return {
    statusCode: 200,
    body: JSON.stringify({
      message: "Hello there"
    }),
  };

  // Use this code if you don't use the http event with the LAMBDA-PROXY integration
  // return { message: 'Go Serverless v1.0! Your function executed successfully!', event };
};


module.exports.register = async (event, context, callback) => {

  let poolData =  {
      UserPoolId : 'xxxxx', // Your user pool id here
      ClientId : 'xxxxxxx' // Your client id here
  } // the user Pool Data

  let userPool = new AmazonCognitoIdentity.CognitoUserPool(poolData);

  let attributeList = [];

  let dataEmail = {
      Name : 'email',
      Value : '[email protected]'
  };

  let dataName = {
      Name : 'name',
      Value : 'Jack'
  };

  var dataPhoneNumber = {
      Name : 'phone_number',
      Value : '+94234324324234' // your phone number here with +country code and no delimiters in front
  };

  let attributeEmail = new AmazonCognitoIdentity.CognitoUserAttribute(dataEmail);
  let attributeName = new AmazonCognitoIdentity.CognitoUserAttribute(dataName);
  var attributePhoneNumber = new AmazonCognitoIdentity.CognitoUserAttribute(dataPhoneNumber);

  attributeList.push(attributeEmail);
  attributeList.push(attributeName);
  attributeList.push(attributePhoneNumber);

  userPool.signUp('[email protected]', 'H1%23$4jsk', attributeList, null, function(err, result){

    let data = {};

    if (err) {
      callback(null, {
          statusCode: 500,
          body: JSON.stringify({
            status: 'FAIL',
            message: err.message
          }),
        });
      } else {
        let cognitoUser = result.user;

        callback(null, {
          statusCode: 200,
          body: JSON.stringify({
            status: 'SUCCESS',
            message: '',
            data: {
              username: cognitoUser.getUsername(),
              id: result.userSub
            }
          }),
        });
      }
  })

  // Use this code if you don't use the http event with the LAMBDA-PROXY integration
  // return { message: 'Go Serverless v1.0! Your function executed successfully!', event };
};

The warning error as follows:

Serverless: Warning: handler 'register' returned a promise and also uses a callback!
This is problematic and might cause issues in your lambda.

Serverless: Warning: context.done called twice within handler 'register'!

serverless.yml

service: test-auth 
plugins:
  - serverless-offline

provider:
  name: aws
  runtime: nodejs8.10
  stage: dev
  region: us-east-1

functions:
  hello:
    handler: handler.hello
    events:
      - http:
          path: message
          method: get
  register:
    handler: handler.register  
    events:
      - http:
          path: register
          method: post

Any help would be appreciated, Thanks in advance.

EDIT (2019-04-01):

module.exports.register = (event, context) => {

  ...

  userPool.signUp('[email protected]', 'H1%23$4jsk', attributeList, null, function(err, result){

    // for testing purpose directly returning 
    return {
      statusCode: 500,
      body: JSON.stringify({
        status: 'FAIL',
        message: err.message
      })
    }

  })

};

Answer 1

Its exactly what the error message states.

All async functions return promises. module.exports.register = async (event, context, callback) => {}

You are also using the callback by calling

callback(null, {
          statusCode: 500,
          body: JSON.stringify({
            status: 'FAIL',
            message: err.message
          }),
        });

Instead of using the callback, just return the either an error or a valid response.