Helper firebase.firestore query function keeps returning undefined even though I want it to return a boolean

Question

My over all goal is to see if a user has made a session in the past on the site. I want to make a helper function that returns a boolean depending on if a user has created a session or not. I am using firebase.firestore() and firebase.auth() for this project. My plan is to query firestore and get an array of emails that are correspond to all the sessions. Then I will loop through all those emails. If one matches the firebase.auth().currentUser.email then I will return true. If it doesnt match then I will return false.

The function keeps returning undefined. I thought it was because you cannot have a break in a forEach loop which is what I was using before.

How to break out of the querySnapshot forEach loop method?

After I read this post I converted my forEach loop into a regular for loop. This did not fix the problem and it keeps returning undefined instead of a boolean.

  // inputs in a user email
  // outputs a boolean 
  // (t if they have made a session)
  // (f if they haven't made a session)
  function madeSessionHelper (userMail) {
    let res;
    firebase.firestore().collection('sessions')
    .get()
    .then((querySnapshot) => {
      for (let i in querySnapshot.docs) {
        const doc = querySnapshot.docs[i]
        if (doc.data().Goal_Email === userMail) {
          res = true;
          break;
        } else {
          res = false;
          break;
        }
      }
      return res;
    })
  }
  console.log(madeSessionHelper("[email protected]"))