how to authenticate an aws cognito user in the browser

aws-cognito-authentication-demo.babel.js

import AWS from 'aws-sdk/global';
import S3 from 'aws-sdk/clients/s3';
import {
  AuthenticationDetails,
  CognitoUser,
  CognitoUserPool,
} from 'amazon-cognito-identity-js';

const REGION = 'some-string-value';
const USER_POOL_ID = 'some-string-value';
const IDENTITY_POOL_ID = 'some-string-value';
const APP_CLIENT_ID = 'some-string-value';
const POOL_KEY = `cognito-idp.${REGION}.amazonaws.com/${USER_POOL_ID}`;

let Username = 'some-string-value';
let Password = 'some-string-value';

let authenticationDetails = new AuthenticationDetails({
  Username,
  Password
});

let userPool = new CognitoUserPool({
  UserPoolId: USER_POOL_ID,
  ClientId: APP_CLIENT_ID
});

let cognitoUser = new CognitoUser({
  Username,
  Pool: userPool
});

let skateboards = {
  mfaRequired(codeDeliveryDetails) {
    let mfaCode = prompt('MFA code is required!');

    cognitoUser.sendMFACode(mfaCode, mfaRequired);
  },
  newPasswordRequired(userAttributes, requiredAttributes) {
    delete userAttributes.email_verified; // it's returned but not valid to submit

    let newPassword = prompt('A new password is required!');

    cognitoUser.completeNewPasswordChallenge(newPassword, userAttributes, newPasswordRequired);
  }
};

let updateAWSCreds = (jwtToken) => {
  AWS.config.credentials = new AWS.CognitoIdentityCredentials({
    IdentityPoolId: IDENTITY_POOL_ID,
    Logins: {
      [POOL_KEY]: jwtToken
    }
  });
};

let authenticateCognitoUser = async ({mfaRequired, newPasswordRequired} = skateboards) => {
  return new Promise((resolve, reject) => {
    cognitoUser.authenticateUser(authenticationDetails, {
      onSuccess(result) {
        let jwtToken = result.getIdToken().getJwtToken();
        updateAWSCreds(jwtToken);
        resolve();
      },
      onFailure(err) {
        reject(err);
      },
      mfaRequired,
      newPasswordRequired
    });
  });
};

let doSomethingInS3ForExample = async () => {
  await authenticateCognitoUser();

  // now do your stuff
};

doSomethingInS3ForExample();

proof of concept, not proof from bullets

lines 68 and 69 don't resolve the promise. That's bad. I'll update it if I get a chance.