User attribute validation
You can use defineAuth and defineFunction to create a Cognito pre sign-up Lambda trigger 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:
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:
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:
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.