[react-router.v4] Uncaught TypeE

1. 选择 react-router 和 react-router-dom
   只需引用一个，两者之处就是dom多了<Link> <BrowserRouter>这样的 DOM 类组件

2. 报错 Uncaught TypeError: Cannot read property 'location' of undefined

impt 引用时写

import React from "react";
import { render } from "react-dom";

import { BrowserRouter as Router, Route, Link } from "react-router-dom";

const Home = () => {
  return (
    <div>
      <h1>test home</h1>
    </div>
  );
};

const about = () => {
  return (
    <div>
      <h1>test about</h1>
    </div>
  );
};

const topics = () => {
  return (
    <div>
      <h1>test topics</h1>
    </div>
  );
};

render(
  <Router>
    <div>
      <ul>
        <li>
          <Link to="/">首页</Link>
        </li>
        <li>
          <Link to="/about">关于</Link>
        </li>
        <li>
          <Link to="/topics">主题列表</Link>
        </li>
      </ul>

      <Route exact path="/" component={Home} />
      <Route path="/about" component={about} />
      <Route path="/topics" component={topics} />
    </div>
  </Router>,

  document.getElementById("root")
);