[Vuejs]-Vue.js Can't check if the user is signed in computed properties

Without more detail on your Vue.js component code it is difficult to help you.

However, note that the documentation explains that:

The recommended way to get the current user is by setting an observer
on the Auth object

firebase.auth().onAuthStateChanged(function(user) {
  if (user) {
      // User is signed in.   
  } else {
      // No user is signed in.   
  } 
});