Universal rendering

Fusion.js supports universal rendering. Universal rendering means that large parts of the codebase can run on the server (for performing server-side rendering) and on the browser.

In some frameworks, this is only limited to React code. In Fusion.js, the entire application runs in a universal context by default, from React components to middlewares in Fusion.js plugins. This means that plugin registration code only needs to be written once even if it requires server-only or browser-only code (e.g. custom hydration code), and that plugins can activate behavior across the entire lifecycle of an application without the need to configure things in many different parts of the app.

Naturally, you can also write React code once and have that code be automatically server-side rendered as you would expect.

Server-only / browser-only code

It is sometimes desirable to write server-only code, browser-only code, and at times, development-only code. To enable that, Fusion.js provides the __NODE__, __BROWSER__ and __DEV__ global flags. These special flags are statically replaced by the compiler with the appropriate boolean value, depending on which bundle the code is compiled for. Then, unused code gets removed via tree shaking and dead code elimination.

To write code that only runs in the server, wrap your code in the appropriate code fence:

if (__NODE__) {
  // server-side code goes here

To write code that only runs in the browser:

if (__BROWSER__) {
  // client-side code goes here

We recommend that you only use __DEV__ to enhance developer experience with regards to error conditions:

// this conditional gets removed from the browser bundle in production, saving a few bytes
if (__DEV__) {
  throw new Error(
    'The `{options}` argument is required. See the documentation at https://the-docs-website/api-docs/the-package'

You should avoid writing significantly different code branches for different environments since doing so introduces more potential points of failure, and makes it more difficult to reproduce issues with confidence.

We also recommend that you use __DEV__ and avoid using process.env.NODE_ENV === 'production', since Fusion.js provides better static compilation and eslint support for the former.


The ES6 standard specifies that import/export syntax cannot appear inside of an if statement of conditional expressions, but Fusion.js is still able to intelligently eliminate server-side imports from client-side bundles, thanks to tree-shaking.

Consider the example below:

import fs from 'fs';

if (__NODE__) fs.readFileSync('package.json');

The compiler removes the fs.readFileSync() call from the browser bundle because the if (__NODE__) code fence evaluates to false, making the code branch unreacheable.

The import statement is outside of the code fence, but it is also removed because the compiler infers that it's also dead code, because no code paths ever use fs in this file for the browser bundle!

Server-side side effects in dependencies

On some rare occasions, poorly written server-side packages might incur top-level side-effects. In those cases, the compiler becomes unable to treeshake the misbehaving dependency in the browser bundle, and compilation typically fails due to unpolyfilled server dependencies.

A simple way to avoid this issue is to simply load the module dynamically via a good old CommonJS require call.

// before
import foo from 'misbehaving-dependency';

// after
const foo = __NODE__ && require('misbehaving-dependency');

Now the code follows the basic dead code elimination rules and the browser bundle will be compiled as expected.


Fusion.js provides an eslint-config-fusion configuration that issues contextual warnings depending on whether code is server-side or client-side.

yarn add eslint-config-fusion

To enable it, add it to your .eslintrc.js:

module.exports = {
  extends: [require.resolve('eslint-config-fusion')],

Now ESLint will complain if you inadvertedly forget to code-fence:

// we didn't code fence this browser-specific code, so it would also try to run in the server. Thus, eslint complains
window.addEventListener('load', () => {});

// after we code-fence, the eslint warning goes away
if (__BROWSER__) {
  window.addEventListener('load', () => {});

You can also mark an entire file as server-only or browser-only:

/* eslint-env node */
/* eslint-env browser */

This pattern is useful if a plugin has vastly different implementations on the server and the browser:

// plugin-entry-point.js
import server from './server';
import client from './client';

export default __NODE__ ? server : client;

// server.js
/* eslint-env node */
export default serverCodeGoesHere;

// server.js
/* eslint-env browser */
export default browserCodeGoesHere;

Disabling server-side rendering

Sometimes it is desirable to avoid server-side rendering. To do that, register a custom render function on the RenderToken on the server. However, instead of disabling SSR entirely, it is probably better to create components that only render in the browser.

// src/main.js
import {RenderToken} from 'fusion-core';
import App from 'fusion-react';
// ...

if (__NODE__) {
  app.register(RenderToken, () => {
    return '<div id="root"></div>';

Client-rendered components

import {split} from 'fusion-react';

const ClientRenderedOnly = split({
  defer: true,
  load: () => import('./browser-only-component.js'),
  LoadingComponent: () => <div>SSR placeholder...</div>,
  ErrorComponent: () => <div>Error</div>,

<ClientRenderedOnly />;