Server-side variables to Client-Side with React-Engine and Express

Question

Server-side variables to Client-Side with React-Engine and Express

15.5k Views Asked by Ant At 08 November 2025 at 06:44

I'm reasonably new to React/React-Engine. I have a config on the server-side that I need to pass certain values of to client-side however I have a dependency on NODE_ENV in order to get the correct config out.

var config = {
    local: { ... }
    production: { ...}
}

module.exports = config[process.env.NODE_ENV]

Which works just fine server-side, however since I need to reference some of values contained in these objects on the client-side so I can't require(./config); in my React JSX.

Is there any easy way to pass this stuff into React? At the end of the day, I'd be happy if I could just pass the 'config' straight into React somehow without even having to worry about NODE_ENV on the client-side.

Thanks

Original Q&A

There are 2 best solutions below

Tyler On 03 April 2017 at 18:09

An alternative to react-engine that works well with express (and any other view rendering node framework) is react-helper (https://github.com/tswayne/react-helper). It pretty much handles everything you need to do to render react components for you in any node framework. You just make an entry point (js file) for webpack (it can generate your webpack config for you) and add a line to your controller and view and your component will render on that page. You can also pass data to your react component from express and when the component binds in your browser it will have access to that server-side data.

const component = reactHelper.renderComponent('MyComponent', {prop: config.prop}) res.render('view-to-render', {component})

There is also express middleware for react-helper (https://github.com/tswayne/express-react-helper) that allows you to add context that is available to all views, which is convenient for config data.

app.use(expressReactHelper.addToReactContext({configProp: config.foo}))

**Guilherme Rodrigues** · Accepted Answer

The most common way to pass data from the server to client before rendering is to embed it in a global JavaScript variable on the page where your React is rendering.

So, for example, in the middleware where you're actually rendering some template which includes your <script> tags with your React app, you could add the info and grab it on the template:

var config = require('../config-from-somewhere');
app.get('/', function (req, res) {
  res.render('index', {config: JSON.stringify(config)});
});

And an example mustache template:

<html>
<body>
<script>
  window.CONFIG = JSON.parse({{{config}}});
</script>
<script src="my-react-app.js"/> 
</body>
</html>

HOWEVER apparently react-engine already provides its own way to send data do the client-side:

Data for component rendering

The actual data that gets fed into the component for rendering is the renderOptions object that express generates.

https://github.com/paypal/react-engine#data-for-component-rendering

As you can see in this example, the movies json is simply being passed into render:

app.get('*', function(req, res) {
  res.render(req.url, {
    movies: require('./movies.json')
  });
});

And then, by the grace of the framework's magic, probably on this line, the information is provided for your components and then the List uses it from props.movies.

module.exports = React.createClass({
  displayName: 'List',

  render: function render() {
    return (
      <div id='list'>
        <h1>Movies</h1>
        <h6>Click on a movie to see the details</h6>
        <ul>
          {this.props.movies.map(function(movie) {
            return (
              <li key={movie.id}>
                <Router.Link to={'/movie/' + movie.id}>
                  <img src={movie.image} alt={movie.title} />
                </Router.Link>
              </li>
            );
          })}

        </ul>
      </div>
    );
  }
});

So, basically add your config to your render call and it should be available in your component's props.

And for the very curious:

Indeed, as we can see on this line onwards, the engine merges renderOptions and res.locals and finally passes that down to React.

// create the data object that will be fed into the React render method.
// Data is a mash of the express' `render options` and `res.locals`
// and meta info about `react-engine`
var data = merge({
  __meta: {
    // get just the relative path for view file name
    view: null,
    markupId: Config.client.markupId
  }
}, omit(options, createOptions.renderOptionsKeysToFilter));

And:

return React.createElement(Component, merge({}, data, routerProps));

Server-side variables to Client-Side with React-Engine and Express

There are 2 best solutions below

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