exports.authCognitoUser = function(theUserPoolID, theClientID, userName, userPassword) {

    var authenticationData = {
        Username : userName,
        Password : userPassword
    };

    var authenticationDetails = new AWSCognito.AuthenticationDetails(authenticationData);

    var poolData = { UserPoolId : theUserPoolID,
        ClientId : theClientID
    };
    var userPool = new AWSCognito.CognitoUserPool(poolData);

    var userData = {
        Username : userName,
        Pool : userPool
    };

    console.log('authentication details: ',authenticationDetails);

    var cognitoUser = new AWSCognito.CognitoUser(userData);

    cognitoUser.authenticateUser(authenticationDetails, {
        onSuccess: function (result) {
            console.log('access token + ' + result.getAccessToken().getJwtToken());
            console.log('id token + ' + result.getIdToken().getJwtToken());
            console.log('refresh token + ' + result.getRefreshToken().getToken());
            return result;
        },
        onFailure: function(err) {
            console.log(err.message || JSON.stringify(err));
            return err;
        },

    });

}

authentication details:  AuthenticationDetails {
  validationData: {},
  authParameters: {},
  username: 'thesmarterstuff',
  password: 'passW0rd!’ }