javascript - 为什么 Flow 不能用 document.getElementById(...) 调用 ReactDOM.render

Question

我在下面的 Flow 类型检查中遇到了这个错误。

Cannot call ReactDOM.render with document.getElementById(...) bound to container because null [1] is
incompatible with Element [2].

     src/index.js
      26│       </Switch>
      27│     </ScrollToTop>
      28│   </BrowserRouter>
      29│ </Provider>, document.getElementById("root"));
      30│

     /private/tmp/flow/flowlib_174a8121/dom.js
 [1] 646│   getElementById(elementId: string): HTMLElement | null;

     /private/tmp/flow/flowlib_174a8121/react-dom.js
 [2]  18│     container: Element,

代码如下。

// @flow
"use strict";
import React from "react";
import ReactDOM from "react-dom";
import {createStore, applyMiddleware} from "redux";
import {Provider} from "react-redux";
import {BrowserRouter, Switch, Route} from "react-router-dom";
import Home from "./components/home";
import Detail from "./components/detail";
import LevelOfGame from "./components/level-of-game";
import NotFound from "./components/not-found";
import ScrollToTop from "./components/scroll-to-top";

import reducers from "./reducers";

const createStoreWithMiddleware = applyMiddleware()(createStore);

ReactDOM.render(<Provider store={createStoreWithMiddleware(reducers)}>
  <BrowserRouter>
    <ScrollToTop>
      <Switch>
        <Route exact path="/" component={Home}/>
        <Route path="/detail/:detailId" component={Detail}/>
        <Route path="/level-of-game" component={LevelOfGame}/>
        <Route path="*" component={NotFound} status={404}/>
      </Switch>
    </ScrollToTop>
  </BrowserRouter>
</Provider>, document.getElementById("root"));

我相信我必须以某种方式在 getElementById 中指定类型。

所以我通过将 document.getElementById("root"); 存储在具有类型规范的常量变量中来修复错误:

const root: any = document.getElementById("root");

错误已修复，我希望这对其他人有用，但我很想了解导致此错误的原因。谁能告诉我这是什么？

Answer 1

Aleksey L.在评论中首先得到这个，我想把这个信息带到答案级别以便于视觉扫描。

Flow 让您知道调用 document.getElementById("root"); 可以返回 null，在这种情况下应用程序将完全崩溃。因此，让我们防范这种情况:

const root = document.getElementById('root')

if (root !== null) {
  ReactDOM.render(<App /> , root)
}

当然，考虑到您很可能会控制要渲染到的 HTML，这可能会让人觉得有点烦人。