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How to set up A/B tests in Node.js (Express)

Feb 02, 2024

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

1. Create a basic Express app

First, ensure Node.js is installed. Then, open your command line, create a new folder for our tutorial, install Express, and create a new file server.js:

Terminal
mkdir express-ab-test
cd express-ab-test
npm i express
touch server.js

Next, add the following code to server.js:

server.js
const express = require('express');
const app = express();
const port = 3000;


app.get('/', (req, res) => {
  const paragraphText = 'Placeholder text'
  const htmlContent = `
    <!DOCTYPE html>
    <html>
    <body>
        <h1>Node A/B testing tutorial</h1>
        <p>${paragraphText}</p>
    </body>
    </html>
  `;


  res.send(htmlContent);
})


app.listen(port, () => {
    console.log(`Example app listening on port ${port}`)
})

Run node server.js and navigate to http://localhost:3000 to see your app in action.

Basic Express 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.

To start, install PostHog’s Node library:

Terminal
npm install posthog-node

Next, we initialize our PostHog client in server.js using our project API key and instance address. You can find these in your project settings.

server.js
const express = require('express');
const app = express();
const port = 3000;
const { PostHog } = require('posthog-node');


const posthogClient = new PostHog('<ph_project_api_key>', {
  host: 'https://us.i.posthog.com',
});


// rest of your code

Our PostHog client is now ready to go.

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 requests made to our endpoint as our goal (although in a production app you'll likely use something like number of user sign ups or number of conversions as your goal).

To measure this, we capture a custom event called endpoint_called whenever we receive a request. To do this, update the app.get function in your code:

server.js
// rest of your code


app.get('/', (req, res) => {
  const distinctId = 'placeholder-user-id'
  posthogClient.capture({
    distinctId,
    event: 'endpoint_called',
  });


// rest of your code
})

With this set up, restart your app and then refresh your browser a few times. You should now see the event captured in PostHog.

Events 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:

  1. Name it "My cool experiment".
  2. Set "Feature flag key" to my-cool-experiment.
  3. Under the experiment goal, select the endpoint_called event we created in the previous step.
  4. Use the default values for all other fields.

Click "Save as draft" and then click "Launch".

Experiment setup in PostHog

5. Implement the A/B test code

To implement the A/B test, we:

  1. Change our request handler to async.
  2. Fetch the my-cool-experiment flag using posthog.getFeatureFlag().
  3. Update the paragraph text based on whether the user is in the control or test variant of the experiment.
server.js
// rest of your code


app.get('/', async (req, res) => {
  const distinctId = 'placeholder-user-id'
  const enabledVariant = await posthogClient.getFeatureFlag('my-cool-experiment', distinctId);
  let paragraphText = 'Placeholder text';
  if (enabledVariant === 'control') {
    paragraphText = 'Control variant!';
  } else if (enabledVariant === 'test') {
    paragraphText = 'Test variant!';
  }


  posthogClient.capture({
    distinctId,
    event: 'endpoint_called',
  });
  
  const htmlContent = `
    <!DOCTYPE html>
    <html>
    <body>
        <h1>Node A/B testing tutorial</h1>
        <p>${paragraphText}</p>
    </body>
    </html>
  `;


  res.send(htmlContent);
})


// rest of your code

When you restart your app and refresh the page, you should see the text updated to either Control variant! or Test variant!.

💡 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 or user ID as their distinctId. For logged-out users, assuming they made their request from a browser, you can use values from their request cookies. See an example of this in the server side section of our Remix tutorial

6. Include the feature flag when capturing your event

To ensure our goal metric is correctly calculated for each experiment variant, we need to include our feature flag information when capturing our endpoint_called event.

To do this, we add the $feature/my-cool-experiment key to our event properties:

server.js
app.get('/', async (req, res) => {
  // rest of your code


  // update your posthogClient.capture call to include the feature flag information
  posthogClient.capture({
    distinctId,
    event: 'endpoint_called',
    properties: {
      '$feature/my-cool-experiment': enabledVariant
    }
  });


  // rest of your code
});

Now PostHog is able to calculate our goal metric for our experiment results.

Experiment results in PostHog

Further reading