React/Express - 'Unexpected token <' in call to renderToString()

I believe the issue stems from the fact that requiring babel-register will not work for the file you import it in, but for files that get imported afterwards. So the JSX syntax of <RouterContext will not be picked up by the renderToString method. This has happened to me before and including babel-register for the syntax never felt clean to me anyway.

What I personally have done and many others do is this: readDomServer.renderToString(React.createElement(RoutingContext, props)) using the createElement method in React, you can accomplish the same thing. This will solve your issue.

babel-register doesn't process the file it is called from, see https://stackoverflow.com/a/29425761/1795821

You'll need to put the app.get() called into another file, or use a different method of loading Babel.

So make a new renderReact.js file with

module.exports = function(app) {
  app.get('*', (req, res) => {
    match({ routes: routes, location: req.url }, (err, redirect, props) => {
        const appHtml = reactDomServer.renderToString(<RouterContext {...props}/>)
        res.send(renderPage(appHtml))
    })
  })
}

Then back in your index.js do this instead

let renderReact = require('./renderReact.js');
renderReact(app);