[Vuejs]-Vue js only loops through one element in firebase database

In each iteration of the forEach() you overwrite the items variable with doc.data();

Using the push() method, as follows, should do the trick:

setMovies(state){

        let items = [];

        db.collection('movies').onSnapshot((snapshot) => {

            snapshot.docs.forEach((doc)=>{
                // eslint-disable-next-line no-console
                console.log(doc.data())
                items.push(doc.data())
            });

            state.movies = items;

        })

 }