FirebaseAuth.js

export class FirebaseAuth extends React.Component {
  static propTypes = {
    children: PropTypes.func.isRequired
  }
  auth = firebase.auth()
  state = { user: this.auth.currentUser }
  componentDidMount () {
    this.unsubscribe = this.auth.onAuthStateChanged(user => {
      this.setState({ user })
    })
  }
  componentWillUnmount () {
    this.unsubscribe()
  }
  render () {
    return this.props.children(this.state.user)
  }
}