React on both backend and frontend

Recently I refactored the express.js app that runs my personal website away from Mustache templates into SSR React. If your server is running in Javascript, why not make its templates Javascript as well? Or, Typescript (TSX).

This required a few steps.

  • Creating utility functions sendComponentAsStaticMarkup and sendComponentAsStringAsync (code). These accept a React component class (and, optionally, initial props), and return a (req, res) =>-style Express route handler, using ReactDOMServer.renderToStaticMarkup and ReactDOMServer.renderToString respectively, which prepends a <!DOCTYPE html> to the body and Content-Type: text/html header before sending off the response.

  • In IndexRouter which serves up the landing page, use sendComponentAsStaticMarkup(IndexComponent) in place of res.render which uses some template engine that you’ve configured in Express. This page doesn’t use a React frontend, we’re just rendering React on the backend, so we use staticMarkup.

  • In TwitchChatStatsRouter which serves up the stats app, use sendComponentAsStringAsync(TwitchChatStatsComponent, propsCallback => { .... propsCallback(data); }). This does have a React frontend, so we want to render to string, not static markup. We are providing a function to provide the initial props for the component as well.

  • In TwitchChatStatsComponent, the server-side template, we add <TwitchChatMonitorApp /> (the frontend component) into its container div, and add an “initialState” prop to both TwitchChatStatsComponent and TwitchChatMonitorApp. I.e., if in the frontend loader you call ReactDOM.render(TwitchChatMonitorApp, document.getElementById("container")), then you would change <div id="container"></div> to <div id="container"><TwitchChatMonitorApp initialState={this.props.initialState} /></div>.

  • Change ReactDOM.render to ReactDOM.hydrate in the System.js launcher for the frontend code.

  • In TwitchChatMonitorApp (frontend), in the constructor, set the initial state based on initialState if it exists. For me, I already have an method for this, since it’s called from within the Websocket onmessage handler.

    constructor(props) {
        if (this.props.initialState) {
            this.updateEmoteByChannel(this.props.initialState.colsToTableData, this.props.initialState.emote);
    updateEmoteByChannel = (newData, emote: string) => {
        this.setState({ colsToTableData: newData, emote: emote });
  • Move any WebSocket logic in the constructor of TwitchChatMonitorApp, into componentDidMount. WebSocket doesn’t exist on the server, and componentDidMount only gets called in the browser, so that’s convenient.

  • Add the React frontend repo as a git submodule of the express backend repo. This way you are explicitly documenting the dependency (complete with commit id). This does require you to git clone --resurse-submodules or at least git submodule update --init.

  • Add "jsx": "react", to the backend tsconfig.json.

Written on January 5, 2019