render a react app in express using cherrytree

cherrytree-express-render.js

/**
 * A generic render helper for cherrytree + react + express.
 *
 * To run this example locally:
 * 
 *   $ git clone https://github.com/QubitProducts/cherrytree.git
 *   $ cd cherrytree/examples/server-side-react
 *   $ npm install
 *   $ npm start
 *   $ open http://localhost:8000
 * 
 */

let cherrytree = require('cherrytree')
let { Router } = require('cherrytree-for-react')
let React = require('react')

/**
 * This higher order function will create a render
 * function that can be used as express middleware.
 * 
 * It takes the route map function and the router options
 * as arguments.
 */
module.exports = function (routes, options) {
  return function render (req, res, next) {
    // this is the URL that the browser has requested
    let url = req.url

    // create a cherrytree instance just for this pageview
    // and configure it with the routes
    let router = cherrytree(options).map(routes())
    
    // on the server, we don't want to use the default
    // cherrytree location - instead we use a MemoryLocation
    // which we initialize with the requested url
    let location = new cherrytree.MemoryLocation(url)

    // kick off the routing using the custom location
    let transition = router.listen(location)

    // transition is a promise (or a subclass of a promise)
    // after the transition completes, we render or redirect
    transition
      .then(function () {
        let html = React.renderToString(<Router router={router} />)
        res.send(html)
      }).catch(function (err) {
        if (err.type === 'TransitionRedirected' && err.nextPath) {
          res.redirect(err.nextPath)
        } else {
          next(err)
        }
      })

    // after everything - clean up
    transition.then(cleanup).catch(cleanup)
    function cleanup () {
      router.destroy()
    }
  }
}