---
title: Adding experiment code
sidebar: Docs
showTitle: true
availability:
    free: none
    selfServe: full
    enterprise: full
---

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:

<MultiLanguage>

```js-web
// 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'}})
```

```react
// 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'}})
```

```react-native
// 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
    }
}           
```

```android_kotlin
if (PostHog.getFeatureFlag("experiment-feature-flag-key")  == "variant-name") {
    // do something
}
```

```ios_swift
if (PostHogSDK.shared.getFeatureFlag("experiment-feature-flag-key") as? String == "variant-name") {
    // do something
}
```

```node
const variant = await client.getFeatureFlag('experiment-feature-flag-key', 'user_distinct_id')

if (variant === 'variant-name') {
    // do something
}
```

```python
variant = posthog.get_feature_flag('experiment-feature-flag-key', 'user_distinct_id')

if variant == 'variant-name':
    # Do something
```

```php
$variant = PostHog::getFeatureFlag('experiment-feature-flag-key', 'user_distinct_id')

if ($variant === 'variant-name') {
    // Do something differently for this user
}
```

```ruby
variant = posthog.get_feature_flag('experiment-feature-flag-key', 'user_distinct_id')

if variant == 'variant-name'
    # Do something 
end
```

```go
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 behaviour)
}
if variant == "variant-name" {
    // Do something
}
```

```elixir
{:ok, feature_flag} = PostHog.feature_flag("experiment-feature-flag-key", "user_distinct_id")

if feature_flag.enabled == "variant-name" do
    # Do something
end
```

```dotnet
var featureFlag = await posthog.GetFeatureFlagAsync("experiment-feature-flag-key", "user_distinct_id");

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

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