import React, { Component } from 'react';
  import { ToastContainer, toast } from 'react-toastify';
  import 'react-toastify/dist/ReactToastify.css';
  // minified version is also included
  // import 'react-toastify/dist/ReactToastify.min.css';

  class App extends Component {
    notify = () => toast("Wow so easy !");

    render(){
      return (
        <div>
        <button onClick={this.notify}>Notify !</button>
          <ToastContainer />
        </div>
      );
    }
  }

add toast.configure(); after toast