为什么Flow无法使用document.getElementById(...)调用ReactDOM.render

Question

我在Flow类型检查中得到了以下错误.

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指定类型getElementById.

所以我通过存储document.getElementById("root");一个带有类型规范的常量变量来修复错误:

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

错误是固定的,我希望这对其他人有用,但我很想知道造成这个错误的原因.任何人都可以这么好,告诉我这是什么？

Answer 1

Aleksey L.在评论中首先得到了这个,我希望将这些信息提升到答案级别,以便更轻松地进行视觉扫描.

Flow让你知道调用document.getElementById("root");可以返回,null在这种情况下应用程序将完全崩溃.所以让我们警惕:

const root = document.getElementById('root')

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

当然,考虑到你很可能会控制你正在渲染的HTML,这会让你觉得有些烦人.

Answer 2

虽然科格尔的答案是正确的，但我认为应该让代码更简单并添加一个例外。

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>, // $FlowIgnore
 document.getElementById("root")
);

注意“$FlowIgnore”注释

然后在您的.flowconfig文件中将其添加到“选项”字段中：

suppress_comment= \\(.\\|\n\\)*\\$FlowIgnore