How to set up Astro analytics, feature flags, and more

Nov 28, 2023

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Astro is a frontend JavaScript framework focused on performance and simplifying the creation of content-based sites. It has seen a rapid increase in interest and usage since its release in 2022.

PostHog provides the tools you need to create the best possible Astro app. In this tutorial, we show you how to set them up. We create a basic Astro blog app, add PostHog on both the client and server, capture custom events, and set up feature flags.

Creating your Astro app

First make sure you install a Node version greater than 18. After that, you can run the command below to create your app. Name your app (we choose astro-tutorial), start your new project Empty, install dependencies, choose No for TypeScript, and No for git repository.

Terminal
npm create astro@latest

Once created, go to the src folder and create a layouts folder. In this folder, create a Layout.astro file with some basic HTML code and links to the home and posts pages.

JavaScript
---
// src/layouts/Layout.astro
---
<html lang="en">
    <head>
        <meta charset="utf-8" />
        <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
        <meta name="viewport" content="width=device-width" />
        <meta name="generator" content={Astro.generator} />
        <title>Astro</title>
    </head>
    <body>
        <a href="/">Home</a>
        <a href="/posts/">Posts</a>
        <slot /> 
    </body>
</html>

Next, in the src/pages folder, we create a posts folder and a basic index.astro file with a link to a post we will create.

JavaScript
---
// src/pages/posts/index.astro
import Layout from '../../layouts/Layout.astro';
---
<Layout>
    <h1>Here are all the posts</h1>
    <a href="/posts/first-post">First Post</a>
</Layout>

In the posts folder, we also create a first-post.md file where we write a bit of Markdown as an example. You can customize this or add more posts and links here if you’d like.

md
---
layout: ../../layouts/Layout.astro
---
<!-- src/pages/posts/first-post.md -->
# First post


PostHog is awesome

We also clean up the base index.astro file in the pages folder to use the layout we created.

JavaScript
---
// src/pages/index.astro
import Layout from '../layouts/Layout.astro';
---
<Layout>
    <h1>Home</h1>
</Layout>

Finally, we can run our app with npm run dev to see our full Astro app running locally.

Adding PostHog on the client side

With our app set up, the next step is to add PostHog to it. To start, create a new components folder in the src folder. In this folder, create a posthog.astro file. In this file, add your Javascript Web snippet which you can find in your project settings. Be sure to include the is:inline directive to prevent Astro from processing it, or you will get Typescript and build errors that property 'posthog' does not exist on type 'Window & typeof globalThis'.

JavaScript
---
// src/components/posthog.astro
---
<script is:inline>
  !function(t,e){var o,n,p,r;e.__SV||(window.posthog=e,e._i=[],e.init=function(i,s,a){function g(t,e){var o=e.split(".");2==o.length&&(t=t[o[0]],e=o[1]),t[e]=function(){t.push([e].concat(Array.prototype.slice.call(arguments,0)))}}(p=t.createElement("script")).type="text/javascript",p.async=!0,p.src=s.api_host+"/static/array.js",(r=t.getElementsByTagName("script")[0]).parentNode.insertBefore(p,r);var u=e;for(void 0!==a?u=e[a]=[]:a="posthog",u.people=u.people||[],u.toString=function(t){var e="posthog";return"posthog"!==a&&(e+="."+a),t||(e+=" (stub)"),e},u.people.toString=function(){return u.toString(1)+".people (stub)"},o="capture identify alias people.set people.set_once set_config register register_once unregister opt_out_capturing has_opted_out_capturing opt_in_capturing reset isFeatureEnabled onFeatureFlags getFeatureFlag getFeatureFlagPayload reloadFeatureFlags group updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures getActiveMatchingSurveys getSurveys getNextSurveyStep onSessionId".split(" "),n=0;n<o.length;n++)g(u,o[n]);e._i.push([i,s,a])},e.__SV=1)}(document,window.posthog||[]);
  posthog.init(
    '<ph_project_api_key>',
    {
      api_host:'https://us.i.posthog.com',
      person_profiles: 'identified_only'
    }
  )
</script>

After doing this, go back to your Layout.astro file, import PostHog, and then add it to the head section.

JavaScript
---
// src/layouts/Layout.astro
import PostHog from '../components/posthog.astro'
---
<html lang="en">
    <head>
        <meta charset="utf-8" />
        <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
        <meta name="viewport" content="width=device-width" />
        <meta name="generator" content={Astro.generator} />
        <title>Astro</title>
        <PostHog />
    </head>
    <body>
        <a href="/">Home</a>
        <a href="/posts/">Posts</a>
        <slot /> 
    </body>
</html>

When you go back to your app and reload, PostHog now autocaptures pageviews, button clicks, session replays (if you enable them), and more.

Event autocaptured

Capturing custom events

Beyond autocapture, you can use PostHog to capture any events you want. To showcase this, we start by adding a button to our homepage component.

JavaScript
---
// src/pages/index.astro
import Layout from '../layouts/Layout.astro';
---
<Layout>
    <h1>Home</h1>
    <button class="main">Great site!</button>
</Layout>

We then add a script to select this button, add a click event listener, and then capture a custom event.

JavaScript
---
// src/pages/index.astro
import Layout from '../layouts/Layout.astro';
---
<Layout>
    <h1>Home</h1>
    <button class="main">Great site!</button>
    <script>
        const button = document.querySelector('.main');
        button.addEventListener('click', () => {
            window.posthog.capture('praise_received')
        });
    </script>
</Layout>

When you go back to your app and click the button, you then see a praise_received event in PostHog.

Custom event capture

Setting up a feature flag

To use a feature flag in your app, first, you must create it in PostHog. Go to the feature flags tab, click "New feature flag," add a key (we chose new-button), set the release condition to 100% of users, and click "Save."

With the feature flag, go back to your home page at components/posthog.astro. Add the loaded: argument to your snippet and implement the posthog.onFeatureFlags callback to update the button text depending on the flag value. If enabled, change the innerText of your button.

JavaScript
---
// src/components/posthog.astro
---
<script is:inline>
  !function(t,e){var o,n,p,r;e.__SV||(window.posthog=e,e._i=[],e.init=function(i,s,a){function g(t,e){var o=e.split(".");2==o.length&&(t=t[o[0]],e=o[1]),t[e]=function(){t.push([e].concat(Array.prototype.slice.call(arguments,0)))}}(p=t.createElement("script")).type="text/javascript",p.async=!0,p.src=s.api_host+"/static/array.js",(r=t.getElementsByTagName("script")[0]).parentNode.insertBefore(p,r);var u=e;for(void 0!==a?u=e[a]=[]:a="posthog",u.people=u.people||[],u.toString=function(t){var e="posthog";return"posthog"!==a&&(e+="."+a),t||(e+=" (stub)"),e},u.people.toString=function(){return u.toString(1)+".people (stub)"},o="capture identify alias people.set people.set_once set_config register register_once unregister opt_out_capturing has_opted_out_capturing opt_in_capturing reset isFeatureEnabled onFeatureFlags getFeatureFlag getFeatureFlagPayload reloadFeatureFlags group updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures getActiveMatchingSurveys getSurveys getNextSurveyStep onSessionId".split(" "),n=0;n<o.length;n++)g(u,o[n]);e._i.push([i,s,a])},e.__SV=1)}(document,window.posthog||[]);
  posthog.init(
    '<ph_project_api_key>',
    {
      api_host:'https://us.i.posthog.com',
      person_profiles: 'identified_only',
      loaded: (posthog) => {
        posthog.onFeatureFlags(() => {
          const button = document.querySelector('.main');
          if (posthog.isFeatureEnabled('new-button')) {
            button.innerText = 'The best site ever!';
          }
        });
      }
    }
  )
</script>

When you reload your page, it shows different button text controlled by the PostHog feature flag.

Flag

Adding PostHog on the server side

So far, we have only used PostHog on the client side. We can also set up PostHog on the server side to capture events on page load, evaluate feature flags before page loads, and more. To do this, start by installing the Node SDK by running:

Terminal
npm i posthog-node

In the src folder, create a posthog.js file. This is where we set up the code to create the PostHog Node client using our project API key and instance address.

JavaScript
// src/posthog.js
import { PostHog } from 'posthog-node';


let posthogClient = null;


export default function PostHogClient() {
  if (!posthogClient) {
    posthogClient = new PostHog('<ph_project_api_key>', {
      host: 'https://us.i.posthog.com',
    });
  }
  return posthogClient;
}

To show how we can use this, we import it into our home pages/index.astro file and then capture an event with it.

JavaScript
---
// src/pages/index.astro
import Layout from '../layouts/Layout.astro';
import PostHogClient from '../posthog.js';


const phClient = PostHogClient();
phClient.capture(
    {
        event: 'server_side_event', 
        distinctId: 'ian@posthog.com'
    }
);
---
<!-- The rest of the component -->

Now when we reload the page, PostHog captures a server_side_event.

Identifying users to connect server and client

One issue with our implementation is the hard-coded user distinct ID we are using to capture events. For it to use a real user’s ID, we can either:

  1. Use the distinct ID set by the client side PostHog in the request cookie.
  2. Create a new distinct ID on the server and use identify() to connect it in the client.

We can start by changing the output mode in astro.config.mjs to hybrid so we can access the cookies.

JavaScript
// astro.config.mjs
import { defineConfig } from 'astro/config';


// https://astro.build/config
export default defineConfig({
  output: 'hybrid'
});

Next, we use the Astro.cookies utility to get the cookie using our project API key.

JavaScript
---
// src/pages/index.astro
// ... imports
const projectAPIKey = '<ph_project_api_key>'
const cookie = Astro.cookies.get(`ph_${projectAPIKey}_posthog`)
let distinctId = cookie?.json().distinct_id
// ... rest of code

If we can’t get the distinct_id this way, we can create our own by importing the Node crypto library and using crypto.randomUUID(). This makes our final server code look like this:

JavaScript
---
// src/pages/index.astro
import Layout from '../layouts/Layout.astro';
import PostHogClient from '../posthog.js';
import crypto from 'node:crypto';


const projectAPIKey = 'phc_zr8PzJVYWUgq47uTjJ8Cp7uCXfkGRppRbIUYlHe5w09'
const cookie = Astro.cookies.get(`ph_${projectAPIKey}_posthog`)
let distinctId = cookie?.json().distinct_id
if (!distinctId) {
    distinctId = crypto.randomUUID()
}


const phClient = PostHogClient();
phClient.capture(
    {
        event: 'server_side_event', 
        distinctId: distinctId
    }
);
---
<!-- The rest of the component -->

This solves our problem partially, but the server and client distinct IDs end up disconnected. Your analysis will show events from two different users when they came from one.

To prevent this, we add the distinct ID in a hidden component so we can access it when PostHog loads.

JavaScript
---
// src/pages/index.astro
//... server logic
---
<Layout>
    <h1>Home</h1>
    <button class="main">Great site!</button>
    <p style="display:none" class="did">{distinctId}</p>
    <!-- ... rest of code -->

Finally, in posthog.astro, we add logic to get the distinct ID, check if it’s different, and call posthog.identify() to connect the IDs if so.

JavaScript
---
// src/components/posthog.astro
---
<script>
  !function(t,e){var o,n,p,r;e.__SV||(window.posthog=e,e._i=[],e.init=function(i,s,a){function g(t,e){var o=e.split(".");2==o.length&&(t=t[o[0]],e=o[1]),t[e]=function(){t.push([e].concat(Array.prototype.slice.call(arguments,0)))}}(p=t.createElement("script")).type="text/javascript",p.async=!0,p.src=s.api_host+"/static/array.js",(r=t.getElementsByTagName("script")[0]).parentNode.insertBefore(p,r);var u=e;for(void 0!==a?u=e[a]=[]:a="posthog",u.people=u.people||[],u.toString=function(t){var e="posthog";return"posthog"!==a&&(e+="."+a),t||(e+=" (stub)"),e},u.people.toString=function(){return u.toString(1)+".people (stub)"},o="capture identify alias people.set people.set_once set_config register register_once unregister opt_out_capturing has_opted_out_capturing opt_in_capturing reset isFeatureEnabled onFeatureFlags getFeatureFlag getFeatureFlagPayload reloadFeatureFlags group updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures getActiveMatchingSurveys getSurveys getNextSurveyStep onSessionId".split(" "),n=0;n<o.length;n++)g(u,o[n]);e._i.push([i,s,a])},e.__SV=1)}(document,window.posthog||[]);
  posthog.init(
    '<ph_project_api_key>',
    {
      api_host:'https://us.i.posthog.com',
            person_profiles: 'identified_only',
      loaded: function(posthog) {
        const button = document.querySelector('.main');
        if (posthog.isFeatureEnabled('new-button')) {
            button.innerText = 'The best site ever!';
        }


                // add this code:
        const distinctId = document.querySelector('.did').innerHTML;
        if (posthog.get_distinct_id() && posthog.get_distinct_id() !== distinctId) {
          posthog.identify(distinctId);
        }
      }
    }
  )
</script>

Now, we will have accurate, combined user IDs on both the client and server.

Setting up server side feature flags

When setting up feature flags, you might have noticed that they flicker on the initial load. The delay between the page and PostHog loading causes this. To prevent it, we can evaluate flags on the server side.

To do this, call isFeatureEnabled for the new-button key and distinctID with the Node phClient. Replace the button text with a server variable that depends on the flag and remove the client side flag logic.

JavaScript
---
// src/pages/index.astro
import Layout from '../layouts/Layout.astro';
import PostHogClient from '../posthog.js';
import crypto from 'node:crypto';


const projectAPIKey = '<ph_project_api_key>'
const cookie = Astro.cookies.get(`ph_${projectAPIKey}_posthog`)
let distinctId = cookie?.json().distinct_id
if (!distinctId) {
    distinctId = crypto.randomUUID()
}


const phClient = PostHogClient();
let buttonText = 'Great site!'
if(await phClient.isFeatureEnabled('new-button', distinctId)) {
    buttonText = 'The best site ever!'
}
---
<Layout>
    <h1>Home</h1>
    <button class="main">{buttonText}</button>
    <p style="display:none" class="did">{distinctId}</p>
</Layout>

Lastly, remove the posthog.onFeatureFlags() code we added in posthog.astro:

JavaScript
---
// src/components/posthog.astro
---
<script>
  !function(t,e){var o,n,p,r;e.__SV||(window.posthog=e,e._i=[],e.init=function(i,s,a){function g(t,e){var o=e.split(".");2==o.length&&(t=t[o[0]],e=o[1]),t[e]=function(){t.push([e].concat(Array.prototype.slice.call(arguments,0)))}}(p=t.createElement("script")).type="text/javascript",p.async=!0,p.src=s.api_host+"/static/array.js",(r=t.getElementsByTagName("script")[0]).parentNode.insertBefore(p,r);var u=e;for(void 0!==a?u=e[a]=[]:a="posthog",u.people=u.people||[],u.toString=function(t){var e="posthog";return"posthog"!==a&&(e+="."+a),t||(e+=" (stub)"),e},u.people.toString=function(){return u.toString(1)+".people (stub)"},o="capture identify alias people.set people.set_once set_config register register_once unregister opt_out_capturing has_opted_out_capturing opt_in_capturing reset isFeatureEnabled onFeatureFlags getFeatureFlag getFeatureFlagPayload reloadFeatureFlags group updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures getActiveMatchingSurveys getSurveys getNextSurveyStep onSessionId".split(" "),n=0;n<o.length;n++)g(u,o[n]);e._i.push([i,s,a])},e.__SV=1)}(document,window.posthog||[]);
  posthog.init(
    '<ph_project_api_key>',
    {
      api_host:'https://us.i.posthog.com',
      person_profiles: 'identified_only',
      loaded: function(posthog) {
        // posthog.onFeatureFlags has been removed


        const distinctId = document.querySelector('.did').innerHTML;
        if (posthog.get_distinct_id() && posthog.get_distinct_id() !== distinctId) {
          posthog.identify(distinctId);
        }
      }
    }
  )
</script>

Now when you refresh your page, your flag won’t flicker because the content is sent from the server. This is especially useful for ensuring good user experiences in A/B tests.

Further reading

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