serverRender.js

// fetch data from the api
import React from "react";
import ReactDOMServer from "react-dom/server";
import App from "./src/components/App";
import axios from "axios";
import config from "./config";

const getApiUrl = contestId => {
    if(contestId) {
        return `${config.serverUrl}/api/contests/${contestId}`;
    }
    return `${config.serverUrl}/api/contests`;
}

const getInitialData = (contestId, apiData) => {
    if(contestId) {
      return {
        currentContestId : apiData._id,
        contests : {
          [apiData._id] : apiData
        }
      }
    }
    return {
      contests : apiData.contests
    }
}

const serverRender = (contestId) =>
  axios.get(getApiUrl(contestId))
    .then(resp => {
      const initialData = getInitialData(contestId, resp.data);
      return {
        initialMarkup: ReactDOMServer.renderToString(
          <App initialData={initialData} />
        ),
        initialData: initialData
      }
    });

export default serverRender;