Hi louis030195, I don’t know a good way of doing this. What I usually do is right at the top of the function, if it’s a create event, define what the data is using as :

exports.foo = functions.firestore
  .document('coll/{docId}')
  .onCreate(async (snap, context) => {
    const something = snap.data() as YourType
    // rest of code
})

And if it’s a changey event like onWrite or onUpdate, I use a little helper function:

import { getDataFromChange } from './somewhere/getDataFromChange'
exports.foo = functions.firestore
  .document('coll/{docId}')
  .onUpdate(async (change, context) => {
    const { isCreate, isDelete, isUpdate, beforeData, afterData } = getDataFromChange<YourType>(change)
  // rest of code
})

This getDataFromChangefunction could probs be better…:

// getDataFromChange.ts
import { DocumentSnapshot } from 'firebase-functions/lib/providers/firestore'
import { Change } from 'firebase-functions'
interface ChangeData<T> {
  isDelete: boolean
  isCreate: boolean
  isUpdate: boolean
  beforeData: T | null
  afterData: T | null
}
interface ChangeDataCreate<T> extends ChangeData<T> {
  isDelete: false
  isCreate: true
  isUpdate: false
  beforeData: null
  afterData: T
}
interface ChangeDataDelete<T> extends ChangeData<T> {
  isDelete: true
  isCreate: false
  isUpdate: false
  beforeData: T
  afterData: null
}
interface ChangeDataUpdate<T> extends ChangeData<T> {
  isDelete: false
  isCreate: false
  isUpdate: true
  beforeData: T
  afterData: T
}
export const getDataFromChange = <T = FirebaseFirestore.DocumentData>(change: Change<DocumentSnapshot>) => {
  const { before, after } = change
if (!before.exists) {
    // Create
    const val: ChangeDataCreate<T> = {
      isDelete: false,
      isCreate: true,
      isUpdate: false,
      beforeData: null,
      afterData: after.data() as T
    }
    return val
  } else if (!after.exists) {
    // Delete
    const val: ChangeDataDelete<T> = {
      isDelete: true,
      isCreate: false,
      isUpdate: false,
      beforeData: before.data() as T,
      afterData: null
    }
    return val
  } else {
    // Update
    const val: ChangeDataUpdate<T> = {
      isDelete: false,
      isCreate: false,
      isUpdate: true,
      beforeData: before.data() as T,
      afterData: after.data() as T
    }
    return val
  }
}

Hope that helps! :)