React Router Feature Flags installation

Install with the wizard

Run this command in your project directory to add PostHog to your React Router project. The wizard detects your framework, installs the SDK, and sets up your first events.

Learn more

Or set it up manually by following the rest of this guide.

  1. Install the package

    Required

    Install the PostHog JavaScript library and React SDK using your package manager:

    npm install --save posthog-js @posthog/react

  2. Configure Vite

    Required

    Add posthog-js and @posthog/react to ssr.noExternal in your vite.config.ts to avoid SSR errors:

    vite.config.ts
    // ... imports
    

    export default defineConfig({
      plugins: [tailwindcss(), reactRouter(), tsconfigPaths()],
      ssr: {
        noExternal: ['posthog-js', '@posthog/react'],
      },
    });

  3. Add the PostHogProvider

    Required

    Initialize PostHog and wrap your app with the PostHogProvider in your app/entry.client.tsx file:

    app/entry.client.tsx
    import { startTransition, StrictMode } from "react";
    import { hydrateRoot } from "react-dom/client";
    import { HydratedRouter } from "react-router/dom";
    import posthog from "posthog-js";
    import { PostHogProvider } from "@posthog/react";
    

    posthog.init("<ph_project_token>", {
      api_host: "https://us.i.posthog.com",
      defaults: "2026-08-30",
    });
    

    startTransition(() => {
      hydrateRoot(
        document,
        <PostHogProvider client={posthog}>
          <StrictMode>
            <HydratedRouter />
          </StrictMode>
        </PostHogProvider>,
      );
    });

  4. Send events

    Click around and view a couple pages to generate some events. PostHog automatically captures pageviews, clicks, and other interactions for you.

    If you'd like, you can also manually capture custom events:

    JavaScript
    posthog.capture('my_custom_event', { property: 'value' })

  5. Client-side rendering

    Required

    Basic flag implementation

    if (posthog.isFeatureEnabled('flag-key')) {
        // Do something differently for this user
        // Optional: fetch the payload
        const matchedFlagPayload = posthog.getFeatureFlagResult('flag-key')?.payload
    }

    Multivariate flags

    const matchedFlag = posthog.getFeatureFlagResult('flag-key')
    if (matchedFlag?.variant == 'variant-key') { // replace 'variant-key' with the key of your variant
        // Do something differently for this user
        // Optional: read the payload from the same result
        const matchedFlagPayload = matchedFlag?.payload
    }

  6. Server-side rendering

    Required

    Use posthog-node to evaluate feature flags on the server. You can access PostHog in a React Router loader:

    app/routes/example.tsx
    import { PostHog } from 'posthog-node'
    import type { Route } from './+types/example'
    

    export async function loader({ request }: Route.LoaderArgs) {
        const posthog = new PostHog(process.env.VITE_POSTHOG_PROJECT_TOKEN!, {
            host: process.env.VITE_POSTHOG_HOST
        })
    

        try {
            // use posthog for feature flags here
        } finally {
            await posthog.shutdown()
        }
    }

    Basic flag implementation

    const isFeatureFlagEnabled = await client.isFeatureEnabled('flag-key', 'distinct_id_of_your_user')
    if (isFeatureFlagEnabled) {
        // Your code if the flag is enabled
        // Optional: fetch the payload
        const matchedFlagPayload = await client.getFeatureFlagPayload('flag-key', 'distinct_id_of_your_user', isFeatureFlagEnabled)
    }

    Multivariate flags

    const enabledVariant = await client.getFeatureFlag('flag-key', 'distinct_id_of_your_user')
    if (enabledVariant === 'variant-key') {  // replace 'variant-key' with the key of your variant
        // Do something differently for this user
        // Optional: fetch the payload
        const matchedFlagPayload = await client.getFeatureFlagPayload('flag-key', 'distinct_id_of_your_user', enabledVariant)
    }

  7. Running experiments

    Optional

    Experiments run on top of our feature flags. Once you've implemented the flag in your code, you run an experiment by creating a new experiment in the PostHog dashboard.

  8. Next steps

    Recommended

    Now that you're evaluating flags, continue with the resources below to learn what else Feature Flags enables within the PostHog platform.

    ResourceDescription
    Creating a feature flagHow to create a feature flag in PostHog
    Adding feature flag codeHow to check flags in your code for all platforms
    Framework-specific guidesSetup guides for React Native, Next.js, Flutter, and other frameworks
    How to do a phased rolloutGradually roll out features to minimize risk
    More tutorialsOther real-world examples and use cases

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