Best way to get Firebase user on each page load / component (Vue)

Question

I need to get the current Firebase authenticated user on each component load in Vue. At the moment I have this in each beforeMount():

firebase.auth().onAuthStateChanged((user) => {
  if (user) {
    // User is signed in, see docs for a list of available properties
    // https://firebase.google.com/docs/reference/js/firebase.User
  } else {
    // User is signed out
    // ...
  }
})

I tried simplifying this to:

var user = firebase.auth().currentUser

It didn't work. Why doesn't it work, when the auth state hasn't changed? I'm confused by this. Is the first method the best way to accomplish this?