[Vuejs]-Inserting into Firebase Realtime Database overwrites data instead of appending, how do I get it to append instead of overwriting in js?

Every time you call set with a reference, it replaces any existing data at that ref.

If you want to create a new unique child nodes under the ref, use push instead of set.

push(userRef, data)

Also see the Firebase documentation on appending to a list of data.

Using an array notation like you’re trying won’t work unless you first read the existing data from the location. Trying to do this is an antipattern in Firebase though, so I also recommend reading Best Practices: Arrays in Firebase.