How to set up A/B tests in Astro

Lior Neu-ner

Jan 29, 2024

A/B tests help you make your Astro app better by enabling you to compare the impact of changes on key metrics. To show you how to set one up, we create a basic Astro app, add PostHog, create an A/B test, and implement the code for it.

1. Create an Astro app

First, ensure Node.js is installed (version 18.0 or newer). Then, create a new Astro app:

Terminal

npm create astro@latest

When prompted in the command line, name your new project directory (we chose astro-ab-test), start your new project Empty, choose No for TypeScript, install dependencies, and No for git repository.

Next, replace the code in src/pages/index.astro with a simple heading and button:

index.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>
        <h1>Astro A/B tests</h1>
        <button class="main-button">Click me!</button>
    </body>
</html>

Run npm run dev and navigate to http://localhost:4321 to see your app in action.

Basic Astro app

2. Add PostHog to your app

With our app set up, it’s time to install and set up PostHog. If you don't have a PostHog instance, you can sign up for free.

Once done, go back to your Astro project and create a new components folder in the src folder. In this folder, create a posthog.astro file

Terminal

cd ./src
mkdir components
cd ./components
touch posthog.astro

In this file, add your Web snippet which you can find in your project settings.

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.crossOrigin="anonymous",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',
    defaults: '2025-05-24',
  })
</script>

The next step is to a create a Layout where we will use posthog.astro. Create a new folder layouts in src and then a new file Layout.astro:

Terminal

cd .. && cd .. # move back to your base directory if you're still in src/components/posthog.astro
cd ./src
mkdir layouts
cd ./layouts
touch Layout.astro

Add the following code to Layout.astro:

Layout.astro

---
import PostHog from '../components/posthog.astro'
---
<head>
    <PostHog />
</head>

Lastly, update index.astro to use the new Layout:

index.astro

---
import Layout from '../layouts/Layout.astro';
---
<Layout>
    <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>
            <h1>Astro A/B tests</h1>
            <button class="main-button">Click me!</button>
        </body>
    </html>
</Layout>

Once you’ve done this, reload your app and click the button a few times. You should see events appearing in the PostHog events explorer.

3. Capture a custom event

The first part of setting up our A/B test in PostHog is setting up the goal metric. We'll use the number of clicks on the button as our goal.

To measure this, we capture a custom event home_button_clicked when the button is clicked. To do this, update the code in posthog.astro to add a <script> and call posthog.capture() when the button is clicked.

index.astro

---
import Layout from '../layouts/Layout.astro';
---
<Layout>
    <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>
            <h1>Astro A/B tests</h1>
            <button class="main-button">Click me!</button>

            <script>
                const button = document.querySelector('.main-button');
                button.addEventListener('click', () => {
                    window.posthog.capture('home_button_clicked')
                });
            </script>   
        </body>
    </html>
</Layout>

With this set up, refresh your app and click the button a few times to see the event captured in PostHog.

4. Create an A/B test in PostHog

Next, go to the A/B testing tab and create an A/B test by clicking the New experiment button. Add the following details to your experiment:

Name it "My cool experiment".
Set "Feature flag key" to my-cool-experiment.
Use the default values for all other fields.
Click Save as draft.

Once created, set the primary metric to a trend of home_button_clicked and then click Launch.

5. Implement the A/B test code

When it comes to implementing our experiment code, there are two options:

Client-side rendering
Server-side rendering

We'll show you how to implement both.

Client-side rendering

To implement the A/B test, we use the posthog.onFeatureFlags callback to update the button text based on whether the user is in the control or test variant of the experiment.

Update the code in /components/posthog.astro to implement posthog.onFeatureFlags code inside PostHog's loaded callback:

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.crossOrigin="anonymous",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',
    defaults: '2025-05-24',
    loaded: (posthog) => {
      posthog.onFeatureFlags(() => {
        const button = document.querySelector('.main-button');
        if (posthog.getFeatureFlag('my-cool-experiment') === 'control') {
          button.innerText = 'Control variant';
        } else if (posthog.getFeatureFlag('my-cool-experiment') === 'test') {
          button.innerText = 'Test variant';
        }
      });
    }
  })
</script>

Now if you refresh your app, you should see the button text updated to either Control variant or Test variant. Users are automatically split between the two, PostHog continues to track button clicks, and you can view the results of the A/B test in PostHog.

Server-side rendering

Notice that when you refresh the page, the button text flickers between Click me! and Control/Test variant. This is because it takes time for PostHog to load and make the feature flag request.

Server-side rendering is a way to avoid this. This fetches the feature flag before the page loads on the client.

To set this up, we must install and use PostHog’s Node library (because we are making server-side requests).

Terminal

npm install posthog-node

In the src folder, create a posthog-node.js file. This is where we set up the code to create the PostHog Node client. You can find both your API key and instance address in your project settings.

src/posthog-node.js

import { PostHog } from 'posthog-node';

let posthogClient = null;

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

Next, we import posthog-node.js into pages/index.astro. Then we use it to fetch the feature flag and update the button text:

index.astro

---
import Layout from '../layouts/Layout.astro';
import PostHogNode from '../posthog-node.js';

let buttonText = 'No variant'
try {
  const distinctId = 'placeholder-user-id'
  const enabledVariant = await PostHogNode().getFeatureFlag('my-cool-experiment', distinctId);
  if (enabledVariant === 'control') {
        buttonText = 'Control Variant';
    } else if (enabledVariant === 'test') {
        buttonText = 'Test Variant';
    }
} catch (error) {
  buttonText = 'Error';
}
---
<Layout>
    <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>
            <h1>Astro A/B tests</h1>
            <button class="main-button">{buttonText}</button>

            <script>
                const button = document.querySelector('.main-button');
                button.addEventListener('click', () => {
                    window.posthog.capture('home_button_clicked')
                });
        </script>   
        </body>
    </html>
</Layout>

Lastly, you can remove the code in posthog.astro which we added for the client-side rendering of feature flags:

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.crossOrigin="anonymous",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'
    defaults: '2025-05-24',
    // the "loaded" argument has been removed
  })
</script>

Now, when you refresh the page, the button text is already set when the page loads.

Setting the correct `distinctId`

You may notice that we set distinctId = 'placeholder-user-id' in our flag call above. In production apps, to ensure you fetch the correct flag value for your user, distinctId should be set to their unique ID.

For logged-in users, you typically use their email as their distinctId. However, for logged-out users, you can use the distinct_id property from their PostHog cookie:

index.astro

---
import Layout from '../layouts/Layout.astro';
import PostHogNode from '../posthog-node.js';

const projectAPIKey = '<ph_project_api_key>';
const cookie = Astro.cookies.get(`ph_${projectAPIKey}_posthog`);
let buttonText = 'No variant'
if (cookie && cookie.json().distinct_id) {
    try {
        const distinctId = cookie.json().distinct_id;
        const enabledVariant = await PostHogNode().getFeatureFlag('my-cool-experiment', distinctId);
        if (enabledVariant === 'control') {
            buttonText = 'Control Variant';
        } else if (enabledVariant === 'test') {
            buttonText = 'Test Variant';
        }
    } catch (error) {
        buttonText = 'Error';
    }
}
---

<!-- reset of your code -->

Note that Astro.request.headers is not available for static sites. If you want to access the request cookies, you need to set your output to server or hybrid in astro.config.mjs:

astro.config.mjs

import { defineConfig } from 'astro/config';

// https://astro.build/config
export default defineConfig({
  output: "server",
});

How to set up A/B tests in Astro

Contents

1. Create an Astro app

2. Add PostHog to your app

3. Capture a custom event

4. Create an A/B test in PostHog

5. Implement the A/B test code

Client-side rendering

Server-side rendering

Setting the correct `distinctId`

Further reading

Product for Engineers

Community questions

How to set up A/B tests in Astro

Contents

Contents

1. Create an Astro app

2. Add PostHog to your app

3. Capture a custom event

4. Create an A/B test in PostHog

5. Implement the A/B test code

Client-side rendering

Server-side rendering

Setting the correct distinctId

Further reading

Product for Engineers

Community questions

Setting the correct `distinctId`