React SSR Fork

React SSR Fork facilitates rendering different content on server and client side, and solves hydration mismatch issue.

Features

• Provides <Client> component to render client side content, and <Server> for server side
• Automatically solves the hydration issue of mismatched client and server side content
• Prevents unecessary client side re-renders when solving hydration mismatch
• Requires minimum setup and is easy to use.

Installation

Install the library:

npm install --save react-ssr-fork

Usage

Wrap your main application component inside <ForkProvider>. This component contains the business logic of solving hydration mismatch issue, also provides context for <Client> and <Server> components.

1. Your client side entry might look like this (<ForkProvider> wrapping is obligatory):

// client.jsx
import React from 'react';
import ReactDOM from 'react-dom';
import { ForkProvider } from 'react-ssr-fork';
import App from 'your-components/App';

ReactDOM.hydrate(
  <ForkProvider>
    <App />    
  </ForkProvider>,
  document.getElementById('root')
);

2. And your server side entry file could look this way (<ForkProvider> wrapping is obligatory):

// server.jsx
import React from 'react';
import { renderToString } from 'react-dom/server';
import { ForkProvider } from 'react-ssr-fork';
import App from 'your-components/App.jsx';

const content = renderToString(
  <ForkProvider>
    <App />
  </ForkProvider>
);
// ...

3. Then use <Client> component to render specifically client side content, and respectively <Server> to render server side content. Your <App> component might look like this:

// App.jsx
import React from 'react';
import { Client, Server } from 'react-ssr-fork';

export default function App() {
  return (
    <div>
      <Client>
        I run on client
      </Client>
      <Server>
        I run on server
      </Server>
    </div>
  );
}

Here's what happens when you run the application:

• On server side <App> simply renders <div>I run on server</div>
• On client side during hydration <App> renders <div>I run on server</div> (to match the content sent from server), then right away re-renders to actual client content <div>I run on client</div>.

Requirements

• On both client and server sides <Client> and <Server> must to be wrapped inside <ForkProvider>
• Requires React 16.3 and above

Why not use this.state.isCient approach?

ReactDOM.hydrate() documentation suggests solving hydration mismatch warning using two-pass rendering and this.state.isClient flag. Here's a simple imlementation:

import React, { Component } from 'react';

class Message extends Component {
  state = { isClient: false }

  render() {
    return (
      <div>
        {this.state.isClient ? 'I run on client' : 'I run on server'}
      </div>
    );
  }

  componentDidMount() {
    this.setState({ isClient: true });
  }
}

ReactDOM.hydrate(
  <Message />,
  document.getElementById('root')
);

On server side <Message> renders <div>I run on server</div>.

On client side during initial render (hydration) this.state.isClient is false, so <Message> outputs <div>I run on server</div>. This output matches the one sent from server side, and React doesn't throw any mismatch warnings during hydration.
Right away after mounting of <Message>, componentDidMount() method is invoked and this.state.isClient becomes true. As result <Message> renders <div>I run on client</div>.

As described above, solving hydration mismatch using two-pass rendering works.

But there is a drawback. If a new instance of <Message> is rendered on the client side on later stages, two-pass rendering will be applied even if that's unnecessary. This makes your component slower. That's the problem solved by React SSR Fork library, which prevents unnecessary two-pass rendering.

Unit testing

React SSR Fork components faciliate unit testing of your components.

<ForkProvider> accepts a special boolean prop canUseDom meant to indicate manually the client or server environment in your unit tests. For example:

import React from 'react';
import { mount } from 'enzyme';

import { ForkProvider } from 'react-ssr-fork';

function App() {
  return (
    <div>
      <Client>
        I run on client
      </Client>
      <Server>
        I run on server
      </Server>
    </div>
  );
}

describe('<App>', function () {
  test('renders client content on client side', function () {
    const wrapper = mount(
      // Client environment: canUseDom=true
      <ForkProvider canUseDom={true}>
        <App />
      </ForkProvider>
    );
    expect(wrapper.contains('I run on client')).toBe(true);
    expect(wrapper.contains('I run on server')).toBe(false);
  });

  test('renders server content on server side', function () {
    const wrapper = mount(
      // Server environment: canUseDom=false
      <ForkProvider canUseDom={false}>
        <App />
      </ForkProvider>
    );
    expect(wrapper.contains('I run on client')).toBe(false);
    expect(wrapper.contains('I run on server')).toBe(true);
  });
});

License

Licensed under MIT