title: What to return from a firebase callable function?
body: <p>So I'm a little confused about returning promises in firebase callable functions. So when I return a promise directly (e.g as a response to adding a new document) it works but gives me an error in the functions log saying that the Maximum call stack size exceeded but when I add a .then() and return the document it doesn't show that error. What am I doing wrong?</p>
  
  <p>thanks</p>