How to set up React feature flags

Feature flags help you release features and conditional show content in your React apps. This tutorial shows you how to create a basic React app, add PostHog, create a feature flag, and then implement the flag to control content in your app.

Create your React app

First, we create our React app using create-react-app and go into the newly created react-flags folder.

npx create-react-app react-flags
cd react-flags

We then remove the boilerplate code in src/App.js to simplify it to just a title.

// src/App.js
import './App.css';

function App() {
  return (
    <div className="App">
      <h1>Welcome to my React app</h1>
    </div>
  );
}

export default App;

Finally, run npm start and go to http://localhost:3000/ to see our new homepage.

Adding PostHog

Since PostHog handles the management and evaluation of feature flags, we must set it up in our app. To do this, start by installing the posthog-js library to get access to the React SDK.

npm install posthog-js

Once, installed we can import PostHog into src/index.js and set up a client using our project API key and instance address from our project settings. We can wrap our app in the React PostHogProvider to access PostHog in any component.

import React from 'react';
import ReactDOM from 'react-dom/client';
import './index.css';
import App from './App';
import posthog from 'posthog-js'
import { PostHogProvider } from 'posthog-js/react'

posthog.init('<ph_project_api_key>', {
  api_host: 'https://us.i.posthog.com',
})

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
  <React.StrictMode>
    <PostHogProvider client={posthog}>
      <App />
    </PostHogProvider>
  </React.StrictMode>
);

Once done, go back to your app running locally, refresh, and you should see an event autocaptured into your PostHog instance.

Creating a feature flag

With PostHog set up, your React app is ready for feature flags. To create one, go to the feature flags tab in PostHog and click "New feature flag." Enter a flag key (like cool-react-homepage), set the release condition to roll out to 100% of users, and press "Save."

You can customize your conditions with percentage and person or group properties to fit your needs.

Adding our feature flag

Once created, we can add our feature flag to our React app. We do this using the useFeatureFlagEnabled hook to conditionally show new content in our component.

// src/App.js
import './App.css';
import { useFeatureFlagEnabled } from 'posthog-js/react';

function App() {
  
  const flagEnabled = useFeatureFlagEnabled('cool-react-homepage');

  return (
    <div className="App">
      {flagEnabled ? 
        <h1>Welcome to my cool new React app</h1> 
        : 
        <h1>Welcome to my React app</h1>
      }
    </div>
  );
}

export default App;

With the flag enabled, our app now shows "Welcome to my cool new React app."

Want to remove the flicker while loading? Read our tutorial on How to bootstrap feature flags in React and Express.

Using the PostHog feature component

An alternate way to implement feature flags is to use the PostHogFeature component. This simplifies the logic of using flags as well as captures related usage automatically (such as a $feature_view event). We set the old content as the fallback for the component.

// src/App.js
import './App.css';
import { PostHogFeature } from 'posthog-js/react';

function App() {
  
  return (
    <div className="App">
      <PostHogFeature 
        flag='cool-react-homepage' 
        match={true} 
        fallback={<h1>Welcome to my React app</h1>}
      >
        <h1>Welcome to my cool new React app</h1> 
      </PostHogFeature>
    </div>
  );
}

export default App;

These are basic implementations of React feature flags setup. From here, you can set up A/B tests, a public beta program, or canary releases.

Further reading

• Testing frontend feature flags with React, Jest, and PostHog
• How to add popups to your React app with feature flags
• How to set up analytics in React

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