Adding experiment code

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Once you've created your experiment in PostHog, the next step is to add your code:

Step 1: Fetch the feature flag

In your experiment, each user is randomly assigned to a variant (usually either 'control' or 'test'). To check which variant a user has been assigned to, fetch the experiment feature flag. You can then customize their experience based on the value in the feature flag:

// Ensure flags are loaded before usage.
// You only need to call this on the code the first time a user visits.
// See this doc for more details:
posthog.onFeatureFlags(function() {
// feature flags should be available at this point
if (posthog.getFeatureFlag('experiment-feature-flag-key') == 'variant-name') {
// do something
// Otherwise, you can just do:
if (posthog.getFeatureFlag('experiment-feature-flag-key') == 'variant-name') {
// do something
// You can also test your code by overriding the feature flag:
// e.g., posthog.featureFlags.override({'experiment-feature-flag-key': 'test'})

Since feature flags are not supported yet in our Java, Rust, and Elixir SDKs, to run an experiment using these SDKs see our docs on how to run experiments without feature flags. This also applies to running experiments using our API.

Step 2 (server-side only): Add the feature flag to your events

This step is not required for events that are submitted via our client-side SDKs (e.g., JavaScript web, iOS, Android, React, React Native).

For our backend SDKs, with the exception of the Go library, this step is not required if the server has local evaluation enabled and the flag in question has no property filters. In these cases, flag information is automatically appended to every event sent to PostHog.

For any server-side events that are also goal metrics for your experiment, you need to include feature flag information when capturing those events. This ensures that the event is attributed to the correct experiment variant (e.g., test or control).

There are two methods to do this:

Include the property $feature/experiment_feature_flag_name: variant_name when capturing events:

distinctId: 'distinct_id',
event: 'event_name_of_your_goal_metric',
properties: {
'$feature/experiment-feature-flag-key': 'variant-name'

Method 2: Set send_feature_flags to true

The capture() method has an optional argument sendFeatureFlags, which is set to false by default. By setting this to true, feature flag information will automatically be sent with the event.

Note that by doing this, PostHog will make an additional request to fetch feature flag information before capturing the event. So this method is only recommended if you don't mind the extra API call and delay.

distinctId: 'distinct_id_of_your_user',
event: 'event_name',
sendFeatureFlags: true,


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Testing and launching an experiment

Once you've written your code, it's a good idea to test that each variant behaves as you'd expect. If you find out your implementation had a bug after you've launched the experiment, you lose days of effort as the experiment results can no longer be trusted. The best way to do this is adding an optional override to your release conditions . For example, you can create an override to assign a user to the 'test' variant if their email is your own (or someone in your team). To do this: Go to…

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