component.jsx

import React, { Component } from 'react'

export default class componentName extends Component {
  constructor(props) {
    super(props);
    this.state = {
      data: null
    }
  }

  componentDidMount() {
    var es = new EventSource('http://localhost:3001/stream')
    es.onmessage = (e) => {
      this.setState({ data: e.data })
    }
  }

  render() {
    return (
      <div>
        { this.state.data ? this.state.data : "nothing to show here"}
      </div>
    )
  }
}