Firebase Authentication : Lookup a user by Email
I use fetchProvidersForEmail(email) and if the result return as empty array then, this email hasn't been use to sign up.
firebase.auth().fetchProvidersForEmail(email)
.then(providers => {
if (providers.length === 0) {
// this email hasn't signed up yet
} else {
// has signed up
}
});
The new method of creating users with email password returns a value whether the given email address is already in use. See here