Firebase Cloud function : Fetching data from Firestore within a for of loop?

I have users following categories. If a new product is added to the database, I want it to be automatically added to their wishlist through a cloud function.

I've been struggling for 2 days on that and no success :( The problem is with the fetch that returns an empty value. I understand from this thread that it has to do with async await but I did not manage to transform my function accordingly (which has to be v8 and not modular)

Thanks for your help guyz!

Here is my function :

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const db = admin.firestore();

exports.addProductToUserWhenCreated = functions.firestore.document('products/{documentUid}').onCreate((snap, context) => {
  // get the categories of the product (array of category iDs, noting that a product can be in different categories) 
  let productCategories = snap.data().categories
  let productId = snap.id
  let categorySelectedBy

  // For each category, 
  for (const category of productCategories) {
    // get the selectedBy field (array of userIds) => HERE IS THE ISSUE
    const snap = db.doc(`categories/${category}`).get()
    snap.then((doc) => {
      return categorySelectedBy = doc.data().selectedBy;
    })
  
    // Add the productId to the user's selectedProducts (by creating a subcollection in the user's document)
    categorySelectedBy.map(user => {
      db.doc(`users/${user}/selectedProducts/${productId}`).set({
        achievedRate: 0,
      })
    })
  }
});

I think you're pretty close. Because you use a for...of loop, you can actually use await if you mark the entire Cloud Function as async:

// Add async here 👉                                                                                    👇
exports.addProductToUserWhenCreated = functions.firestore.document('products/{documentUid}').onCreate(async (snap, context) => {
  // get the categories of the product (array of category iDs, noting that a product can be in different categories) 
  let productCategories = snap.data().categories
  let productId = snap.id
  let categorySelectedBy

  // For each category, 
  for (const category of productCategories) {
    // get the selectedBy field (array of userIds)
    // await here 👇
    const doc = await db.doc(`categories/${category}`).get()
    const categorySelectedBy = doc.data().selectedBy;
  
    // Add the productId to the user's selectedProducts (by creating a subcollection in the user's document)
    // 👇 Use Promise.all to wait for all writes here
    Promise.all(categorySelectedBy.map(user => {
      return db.doc(`users/${user}/selectedProducts/${productId}`).set({
        achievedRate: 0,
      })
    }))
  }
});