Pre Signup Validation on AWS Cognito

  1. The validation will only live in this particular backend. If we create another app that interacts with the same user pool, we will have to repeat this logic there too.
  2. We’re adding bloat to the request handler, we could keep this handler clean by avoiding unnecessary code if we can.
  1. We can add custom validation when registering a new user to the user pool. In our case, it’s to check if the submitted email is already in use.
  2. The logic is baked into the user pool. Any signup event for this user pool will be validated regardless of where it’s coming from.



const { CognitoIdentityProviderClient, ListUsersCommand } = require("@aws-sdk/client-cognito-identity-provider");exports.handler = async (event, context, callback) => {
// TODO implement
const params = {
UserPoolId: 'us-west-2_rxOJKcUKc',
Filter: `email = "${}"`
const client = new CognitoIdentityProviderClient();
const listUsersCommand = new ListUsersCommand(params);
const data = await client.send(listUsersCommand);if (data?.Users?.length > 0) {
callback(new Error("Email is already taken"), event);
} else {
callback(null, event);


"scripts": {
"build": "rm -rf nodejs && rm && npm install && mkdir nodejs && mv node_modules nodejs && zip -r . && cp ~/Downloads"
  1. Delete the previous nodejs directory and file from the previous build.
  2. Install the packages.
  3. Create a directory called nodejs.
  4. Move the node_modules folder into nodejs.
  5. Zip the current directory into a zip file called
  6. Copy the zip file to the desired location (Optional).


"Version": "2012-10-17",
"Statement": []
"Effect": "Allow",
"Action": "cognito-idp:ListUsers",
"Resource": "arn:aws:cognito-idp:<region>:<account_id>:userpool/<userpool_id>"



UserLambdaValidationException: PreSignUp failed with error Email is already taken.
'$fault': 'client',
'$metadata': {
httpStatusCode: 400,
requestId: '3bc8f968-cbf5-4960-857f-e48daa312870',
extendedRequestId: undefined,
cfId: undefined,
attempts: 1,
totalRetryDelay: 0
__type: 'UserLambdaValidationException'


event.response.autoConfirmUser = true event.response.autoVerifyPhone = true event.response.autoVerifyEmail = true




Full-stack software developer. #WebDev #Programming

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