HowTo: Grant IAM role to Cognito authenticated user

main.js

let response = await login(email, password)
const idToken = response.AuthenticationResult.IdToken
const { identityId, credentials } = await getUserCredentials(idToken)

const s3 = new AWS.S3({ credentials })
response = await s3.putObject({
  Body: 'Hello world',
  Bucket: process.env.S3_BUCKET, 
  Key: `${identityId}/foo.txt`,
  ContentType: 'text/plain'
}).promise()