Getting Started with Posthog Nextjs for Feature Flags and Analytics

Getting started with Posthog Nextjs is a breeze, especially when you have the right tools at your disposal. With Posthog Nextjs, you can easily track user behavior and make data-driven decisions to improve your product.

To begin, you'll need to set up Posthog Nextjs in your Nextjs project. This involves installing the Posthog Nextjs library using npm or yarn, and then initializing the library in your Nextjs app.

Posthog Nextjs provides a range of features that make it easy to get started with feature flags and analytics. For example, you can use the library to track user behavior, segment your users, and create feature flags to control the roll-out of new features.

By following these simple steps, you'll be up and running with Posthog Nextjs in no time, and be able to start making data-driven decisions to improve your product.

Additional reading: Posthog Webflow

Feature Flags are a powerful tool in PostHog Next.js, allowing you to control the visibility and behavior of features in your app. You can load flags right away by bootstrapping them, which means getting the flag data from PostHog in between a request and page load.

To set this up, you'll need to create a middleware.js file in your base directory, which will handle the request and set up the matcher config. Then, use your project API key to get your cookie and check for the distinct ID or create one if it isn’t found.

By bootstrapping flags, you can prevent "flickering" and ensure that features load instantly. This is especially important when users interact with your app, as flags should update in real-time. Note that when using Next.js v12, there's a bug in 12.3.0 that can cause session recording not to work, so make sure to use at least v12.3.4.

Discover more: Why Use Next Js

Bootstrapping feature flags is a great way to ensure that your flags load instantly, especially when properties change. This approach uses the posthog-js library to update flags in real-time.

To bootstrap feature flags, you'll need to get the cookie data and pass it to the PostHog initialization in your providers.js file. This will allow you to rework your page.js file to check that the flag isn't undefined before using its value.

Using cookies directly won't work because they don't update when properties change. Instead, you should use bootstrapping to keep flags up-to-date in the user session. For example, when a user opts into a beta.

When using Next.js v12, be aware that there's a bug in 12.3.0 that can cause session recording not to work. To fix this, use a version of Next.js v12 at least at v12.3.4.

Take a look at this: Nextjs Session

You can use the PostHog provider to work with feature flags in your client-side app. The provider gives you hooks to do this.

To integrate PostHog with your Next.js application, you must install the Posthog-js package using your preferred package manager, like yarn or npm.

You can then access PostHog throughout your Next.js app by using the usePostHog hook.

To use the usePostHog hook, create a feature flag called blog-cta and set it to 100% of users. This will display a call to action on your blog page.

The code we wrote in the client-side rendering section did all of this after the page initially loads. There is a way to remove client-side rendering and have CTA display immediately on page load.

To do this, you need to create a middleware.js file in your base directory, not in the app directory. In this file, you can handle the request and set up the matcher config with a negative lookahead to remove non-page requests.

You can then use your project API key to get your cookie, with the cookie, check for the distinct ID or create one if it isn’t found.

With the flag data, you can create an object to use the client-side bootstrapping and then pass this value as a cookie.

Worth a look: Nextjs Rendering

Client-side rendering is a feature that allows you to run code on the client-side, enabling more interactive and dynamic user experiences.

The PostHog provider is a client component, but it doesn't de-opt client-side rendering because Next.js creates a boundary between server-run and client-run code. This means that server components can still be used alongside the PostHog provider.

Client-side rendering can be used to work with feature flags in your app, allowing you to display or hide certain features based on user interactions.

To create a Next.js app, start by installing Node, then create a new app with Next.js. Select No for TypeScript and Yes for use app router.

Next.js is a popular framework for building server-side rendered and statically generated websites. For this example, we'll be using the default options.

Create a new file in the app folder named providers.js. In this file, import PostHogProvider and initialize PostHog with your project API key and instance address.

You'll need to set up a PostHog provider to track user behavior in your app. This involves creating a new provider component in the providers.js file.

Import your provider component into layout.js and wrap your main app with it. This will enable PostHog tracking throughout your application.

Wrapping your app in the PostHog provider can have a significant impact on client-side rendering. This method allows you to access PostHog throughout your Next.js app using the usePostHog hook.

The PostHog provider gives you access to features like autocapture, custom events, session recording, and more. It also provides hooks like usePostHog and useFeatureFlagEnabled, as well as a PostHog context you can access in any component.

You can use a window object check to initialize PostHog, but this can cause a hydration and/or mismatch error. This is because it happens outside the React lifecycle, meaning it happens earlier and can cause issues.

Wrapping your app in the PostHog provider does not de-opt it to client-side rendering. This is because Next.js creates a boundary between server-run and client-run code, allowing server components to still be used within the provider.

To initialize PostHog, you can use a useEffect hook. This is recommended by the React docs, as it allows you to specify side effects that are caused by rendering itself, rather than by a particular event.

Here's a summary of the benefits of using the PostHog provider:

By using the PostHog provider, you can effectively track user interactions and features in your Next.js app, even with client-side rendering.

Event tracking is a powerful feature in PostHog, allowing you to define and track custom events specific to your application.

You can capture custom events in your components, like a "like" button that includes article details as properties. This can be done by creating a button and connecting it to a function that captures a post liked event with the post title and author.

PostHog enables you to focus on metrics that align with your goals by capturing custom events specific to your app. Whether you're tracking how many times a particular feature is used or monitoring the completion of a key action, you can define events that matter to you and your business.

You can record and replay user sessions, helping you visualize how users interact with your application and identify pain points or areas for improvement. This is made possible by PostHog's ability to track custom events.

Tracking fundamental user interactions such as page views, button clicks, and form submissions provides valuable insights into user behavior. This data allows you to optimize your application based on user interactions, leading to better user experiences and improved application performance.

Asynchronous event tracking is essential for maintaining a smooth user experience and ensuring your application performs optimally. Sending PostHog events asynchronously helps prevent blocking the main thread, ensuring your application remains responsive.

Capturing pageviews in Next.js requires some extra setup, as the app does not reload when moving between pageviews. To solve this, you can capture a custom event when the route changes using next/router in _app.js with useEffect.