Custom exception name instead of UserLambdaValidationException - node.js

Question

I use the Pre sign-up trigger of Cognito to validate the user registration. If some validation fails the function responses with a custom error. The client only receive the UserLambdaValidationException as ErrorCode.
Is there a way to receive the custom error name instead?
Current using sample:
exports.handler = function(event, context, callback) {
function AccountAlreadyExistsError(message) {
this.name = "AccountAlreadyExistsError";
this.message = message;
}
AccountAlreadyExistsError.prototype = new Error();
const error = new AccountAlreadyExistsError("Account is in use!");
callback(error);
};
I want to get AccountAlreadyExistsError in our client instead of UserLambdaValidationException.

Answer 1

You can customise exception headers only if it is a Lambda Proxy, with the API Gateway (x-amzn-errortype header), in any other case you need to parse the exception message to get your custom exception, here is an example how I manage exceptions for Cognito lambda triggers, it's just to show you the idea:
/*
Lambda Code, could be in a Lambda layer
*/
const EXCEPTION_TYPE_DELIMITER = "etype"
const EXCEPTION_MESSAGE_DELIMITER = "emsg";
const beTagged = (tag, txt) => {
return "<" + tag + ">" + txt + "</" + tag + ">";
};
class UserNotAllowedException extends Error {
constructor (message) {
super(beTagged(EXCEPTION_MESSAGE_DELIMITER, message))
Error.captureStackTrace(this, this.constructor);
this.name = beTagged(EXCEPTION_TYPE_DELIMITER, this.constructor.name);
}
}
exports.handler = async (event, context, callback) => {
// Your logic, etc..
throw new UserNotAllowedException("You are blocked :(");
}
Here is the parser
/*
Client Code
*/
const EXCEPTION_TYPE_DELIMITER_REGEX = /<etype>(.*?)<\/etype>/g;
const EXCEPTION_TYPE_UNTAG_REGEX = /<\/?etype>/g;
const EXCEPTION_MESSAGE_DELIMITER_REGEX = /<emsg>(.*?)<\/emsg>/g;
const EXCEPTION_MESSAGE_UNTAG_REGEX = /<\/?emsg>/g;
const untag = (txt, delimiterRegex, untagRegex) => {
return txt.match(delimiterRegex).map(etype => {
return etype.replace(untagRegex,"");
})[0];
};
const resolveException = (exceptionMessage) => {
const exceptionType = untag(exceptionMessage, EXCEPTION_TYPE_DELIMITER_REGEX, EXCEPTION_TYPE_UNTAG_REGEX);
const exceptionMessage = untag(exceptionMessage, EXCEPTION_MESSAGE_DELIMITER_REGEX, EXCEPTION_MESSAGE_UNTAG_REGEX);
// Your logic to determine what exception is the exceptionType
return new YourResolvedException(exceptionMessage);
};
try {
// This guy should throw the exception
return await Auth.signUp(params);
} catch (e) {
// Expected message from Cognito Lambda Trigger
// PreSignUp failed with error <etype>UserNotAllowedException</etype>: <emsg>You are blocked :(</emsg>.
// For example, here your custom exception resolver
throw resolveException(e.message);
}
It can be done in a thousand ways, but the best thing for us would be if the AWS services were able to fully customise the exceptions without so much hassle.
Regards!

Answer 2

I tried it on my end and was able to get the error message in the client. Tested it using AWS CLI, Nodejs & the built-in Cognito UI. I returned the error using:
var error = new Error('something went wrong..!');
callback(error,event);
I got the error UserLambdaValidationException: PreSignUp failed with error something went wrong..! in all my clients. Even callback(error) worked.