Adding experiment code

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

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:

Only getFeatureFlag() counts as an exposure

You must use getFeatureFlag() (or its framework equivalent like useFeatureFlagVariantKey()) to check variants. Other methods like getAllFlags(), getFeatureFlags(), or getFeatureFlagPayload() alone do not record an exposure event. Users evaluated with those methods won't be included in your experiment results.

// 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: /docs/feature-flags/manual#ensuring-flags-are-loaded-before-usage
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.overrideFeatureFlags({ flags: {'experiment-feature-flag-key': 'test'}})
// You can either use the `useFeatureFlagVariantKey` hook,
// or you can use the feature flags component - /docs/libraries/react#feature-flags-react-component

// Method one: using the useFeatureFlagVariantKey hook
import { useFeatureFlagVariantKey } from '@posthog/react'

function App() {
    const variant = useFeatureFlagVariantKey('experiment-feature-flag-key')
    if (variant == 'variant-name') {
        // do something
    }
}

// Method two: using the feature flags component
import { PostHogFeature } from '@posthog/react'

function App() {
    return (
        <PostHogFeature flag='experiment-feature-flag-key' match={'variant-name'}>
            <div>
                <!-- the component to show -->
            </div>
        </PostHogFeature>
    )
}


// You can also test your code by overriding the feature flag:
// e.g., posthog.featureFlags.overrideFeatureFlags({ flags: {'experiment-feature-flag-key': 'test'}})
// With the useFeatureFlag hook
import { useFeatureFlag } from 'posthog-react-native'

const MyComponent = () => {
    const variant = useFeatureFlag('experiment-feature-flag-key')

    if (variant === undefined) {
        // the response is undefined if the flags are being loaded
        return null
    }

    if (variant == 'variant-name') {
        // do something
    }
}
if (PostHog.getFeatureFlag("experiment-feature-flag-key")  == "variant-name") {
    // do something
}
if (PostHogSDK.shared.getFeatureFlag("experiment-feature-flag-key") as? String == "variant-name") {
    // do something
}
const variant = await client.getFeatureFlag('experiment-feature-flag-key', 'user_distinct_id')

if (variant === 'variant-name') {
    // do something
}
variant = posthog.get_feature_flag('experiment-feature-flag-key', 'user_distinct_id')

if variant == 'variant-name':
    # Do something
$variant = PostHog::getFeatureFlag('experiment-feature-flag-key', 'user_distinct_id')

if ($variant === 'variant-name') {
    // Do something differently for this user
}
variant = posthog.get_feature_flag('experiment-feature-flag-key', 'user_distinct_id')

if variant == 'variant-name'
    # Do something 
end
variant, err := client.GetFeatureFlag(posthog.FeatureFlagPayload{
    Key:        "experiment-feature-flag-key",
    DistinctId: "user_distinct_id",
})
if err != nil {
    // Handle error (e.g. capture error and fallback to default behavior)
}
if variant == "variant-name" {
    // Do something
}
{:ok, feature_flag} = PostHog.feature_flag("experiment-feature-flag-key", "user_distinct_id")

if feature_flag.enabled == "variant-name" do
    # Do something
end
Object flagValue = posthog.getFeatureFlag("user_distinct_id", "experiment-feature-flag-key", "control");
String variant = flagValue instanceof String ? (String) flagValue : "control";

if ("variant-name".equals(variant)) {
    // Do something
}
var featureFlag = await posthog.GetFeatureFlagAsync("experiment-feature-flag-key", "user_distinct_id");

if (featureFlag is { VariantKey: "variant-name" })
{
    // Do something
}

To run an experiment using our API (without any SDK), see our docs on how to run experiments without feature flags.

Adding experiment code

Contents

Fetch the feature flag

Community questions

Was this page useful?