Mongo Join String Array

I am writing an application to consume the data in my Mongo database, but I need to process the data in a flat format. My document has a few arrays in it that I would like to turn into a string array, but can't find a way to do so with a mongo query.

Document:

{
  name: 'Hello World',
  tags: ['simple', 'easy']
}

Desired Output:

{
  name: 'Hello World',
  tags: 'simple,easy',
}