Firebase onSnapshot listener: How to handle if doc does not exist yet?

I am building an app where users can tag subjects they are interested in.

I am looking for best practices on how to handle the following issue:

I have a page that mounts an onSnapshot listener during page creation (example page path: This page allows the user to actually add the tag to their feed. First, I need to "watch" whether they add the tag to their feed or check to see if it already exists so that way I can toggle the state of a button icon to let them know that they already have added this tag to their feed.

I thought I'd use the onSnapshot listener to handle this logic like so (simplified for this question):

mounted() {

    loadwatchedTag() {
      const docRef = doc(db, `users/${this.userProfile.uid}/tags`, 'apple') //<-- example tag

      this.unsubscribe = onSnapshot(docRef, {
        next: (doc) => console.log('tag',,
        error: (error) =>
          console.log('watchedTag listener error', error)


The problem is that on page load, I get the following error in console (because the tag does not yet exist):

tag, undefined

So, how to handle this issue on page load if the tag does not yet exist? Should I be doing something like this within the onSnapshot code?

if (docSnap.exists()) {
  console.log("tag exists!",;
} else {
  // will be undefined in this case
  console.log("tag does not exist");
Updated 2022-01-14 11:59:10Z by Dharmaraj
Updated 2022-01-14 11:57:28Z by redshift