Svelte experiments installation

Install the package

Required

Install the PostHog JavaScript library using your package manager:

npm install posthog-js
yarn add posthog-js
pnpm add posthog-js

Initialize PostHog

Required

If you haven't created a root layout already, create a new file called +layout.js in your src/routes folder. Check the environment is the browser, and initialize PostHog if so:

src/routes/+layout.js
import posthog from 'posthog-js'
import { browser } from '$app/environment';
import { onMount } from 'svelte';

export const load = async () => {
  if (browser) {
    posthog.init(
      '<ph_project_api_key>',
      {
        api_host: 'https://us.i.posthog.com',
        defaults: '2025-11-30'
      }
    )
  }

  return
};

SvelteKit layout

Learn more about SvelteKit layouts in the official documentation.

Server-side setup

Optional

Install posthog-node using your package manager:

npm install posthog-node --save
yarn add posthog-node
pnpm add posthog-node
bun add posthog-node

Then, initialize the PostHog Node client where you'd like to use it on the server side. For example, in a load function:

routes/+page.server.js
import { PostHog } from 'posthog-node';

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

  posthog.capture({
    distinctId: 'distinct_id_of_the_user',
    event: 'event_name',
  })

  await posthog.shutdown()
}

Note

Make sure to always call posthog.shutdown() after capturing events from the server-side. PostHog queues events into larger batches, and this call forces all batched events to be flushed immediately.

Implement your experiment

Required

Experiments run on top of our feature flags. You can define which version of your code runs based on the return value of the feature flag.

For client-side experiments, use the JavaScript snippet. For server-side experiments, use the Node.js snippet:

if (posthog.getFeatureFlag('your-experiment-feature-flag') === 'test') {
    // Do something differently for this user
} else {
    // It's a good idea to let control variant always be the default behaviour,
    // so if something goes wrong with flag evaluation, you don't break your app.
}

// Test that it works
posthog.featureFlags.overrideFeatureFlags({ flags: {'your-experiment-feature-flag': 'test'} })

5
Run your experiment
Required
Once you've implemented the feature flag in your code, you'll enable it for a target audience by creating a new experiment in the PostHog dashboard.

Next steps

Recommended

Resource	Description
Creating an experiment	How to create an experiment in PostHog
Adding experiment code	How to implement experiments for all platforms
Statistical significance	Understanding when results are meaningful
Experiment insights	How to analyze your experiment data
More tutorials	Other real-world examples and use cases

Svelte experiments installation

Install the package

Initialize PostHog

Server-side setup

Implement your experiment

Run your experiment

Next steps

Community questions

Was this page useful?