---
title: "User attribute validation"
section: "build-a-backend/functions/examples"
platforms: ["android", "angular", "flutter", "javascript", "nextjs", "react", "react-native", "swift", "vue"]
gen: 2
last-updated: "2025-04-30T20:16:55.000Z"
url: "https://docs.amplify.aws/react/build-a-backend/functions/examples/user-attribute-validation/"
---

You can use `defineAuth` and `defineFunction` to create a [Cognito pre sign-up Lambda trigger](https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-pre-sign-up.html) that extends the behavior of sign-up to validate attribute values.

To get started, create a new directory and a resource file, `amplify/auth/pre-sign-up/resource.ts`. Then, define the function with `defineFunction`:

```ts title="amplify/auth/pre-sign-up/resource.ts"
import { defineFunction } from '@aws-amplify/backend';

export const preSignUp = defineFunction({
  name: "pre-sign-up",
  resourceGroupName: 'auth'
});
```

Next, create the corresponding handler file, `amplify/auth/pre-sign-up/handler.ts`, file with the following contents:

```ts title="amplify/auth/pre-sign-up/handler.ts"
import type { PreSignUpTriggerHandler } from "aws-lambda"

function isOlderThan(date: Date, age: number) {
  const comparison = new Date()
  comparison.setFullYear(comparison.getFullYear() - age)
  return date.getTime() <= comparison.getTime()
}

export const handler: PreSignUpTriggerHandler = async (event) => {
  const birthdate = new Date(event.request.userAttributes["birthdate"])

  // you must be 13 years or older
  if (!isOlderThan(birthdate, 13)) {
    throw new Error("You must be 13 years or older to use this site")
  }

  return event
}
```

Lastly, set the newly created function resource on your auth resource:

```ts title="amplify/auth/resource.ts"
import { defineAuth } from '@aws-amplify/backend';
import { preSignUp } from './pre-sign-up/resource';

export const auth = defineAuth({
  // ...
  triggers: {
    preSignUp
  }
});
```

After deploying the changes, whenever a user attempts to sign up this handler will verify the submitter's age is above 13 years.