How to write file in memory or to file in Lambda aws?

I want to use lambda to get database from dynamodb and generate compute data to cvs file.

Then attach this file with email send to customer.

Step

1. 
   

   Get data from dynamodb. (I know how to do it .)

   
   


2. 
   

   Write to file .CSV ( need help).

   
   

Because lambda doesn't have persisted data. How to write to file

3. Attach .CSV to email and send to customer. ( need help)