import React from 'react'
import firebase from 'firebase'

class MyComponent extends React.Component {
   componentDidMount () {
    this.unsubscribe = firebase.auth().onAuthStateChanged(user => {
      this.setState(user)
    })
   }

   componentWillUnmount () {
     this.unsubscribe()
   }

  render () {
    if (!this.state.user) return <p>You're not logged in!</p>
    return <p>Hello, {this.state.user.email}</p>
  }
}