feat: Support for client-assigned feature flags

[danielbachhuber]

Changes

Adds support for client-assigned feature flags. When bootstrap.clientAssignedFeatureFlags is provided, the specified feature flags will be assigned with values based on the PostHog backend's allocation strategy.

posthog.init('<ph_project_api_key>', {
    bootstrap: {
        clientAssignedFeatureFlags: [
            {
                key: 'my-awesome-experiment-flag',
                variants: {
                    test: 0.5,
                    control: 0.5,
                },
            },
        ],
    },
})

Related PostHog/posthog#26145

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Nov 12, 2024 1:18pm

[github-actions]

Size Change: +20.3 kB (+0.66%)

Total Size: 3.12 MB

Filename	Size	Change
dist/array.full.es5.js	255 kB	+2.29 kB (+0.91%)
dist/array.full.js	357 kB	+2.01 kB (+0.57%)
dist/array.full.no-external.js	356 kB	+2.01 kB (+0.57%)
dist/array.js	172 kB	+2.01 kB (+1.18%)
dist/array.no-external.js	171 kB	+2.01 kB (+1.19%)
dist/main.js	173 kB	+2.01 kB (+1.18%)
dist/module.full.js	357 kB	+2.01 kB (+0.57%)
dist/module.full.no-external.js	356 kB	+2.01 kB (+0.57%)
dist/module.js	172 kB	+2.01 kB (+1.18%)
dist/module.no-external.js	171 kB	+2.01 kB (+1.19%)

ℹ️ View Unchanged

Filename	Size
dist/all-external-dependencies.js	204 kB
dist/dead-clicks-autocapture.js	13 kB
dist/exception-autocapture.js	8.8 kB
dist/external-scripts-loader.js	2.19 kB
dist/recorder-v2.js	113 kB
dist/recorder.js	113 kB
dist/surveys-preview.js	56.7 kB
dist/surveys.js	62.1 kB
dist/tracing-headers.js	1.33 kB
dist/web-vitals.js	10.3 kB

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[dmarticus]

this is interesting work; what's the context here? Is this supporting some sort of customer request?

[dmarticus]

😆

[dmarticus]

we use sha1 in both the python and rust implementations when creating hash keys; any reason why we wouldn't do the same here?

[danielbachhuber]

any reason why we wouldn't do the same here?

@dmarticus Tell me more? I ran into some IE11 linting issue with crypto, but maybe we can actually use it.

[dmarticus]

argh, should've known there's some browser compat issue – there's some prior art (slack, github around separating our bundles into es5 (for things that explicitly need IE11 support) and es6 (for everything else), but I'm not sure what our policy on adding new behavior that explicitly doesn't support IE11 is.

It might be a pain to support it here though, even if crypto is technically "supported" on IE11, it seems like the support pegged to a specific version of that library, and I'm not sure if taking on the overhead of making sure we have IE11 support is worth using the library. Then again, using a standard for crypto is probably safer than rolling our owning hashing algo. TBH – the decision is up to you, I just wanted to provide my 2 cents. I definitely didn't realize this was a browser compat thing beforehand.

[danielbachhuber]

Is this supporting some sort of customer request?

@dmarticus Yep, see https://posthog.slack.com/archives/C07PXH2GTGV/p1731099028903259

[dmarticus]

Is this supporting some sort of customer request?

@dmarticus Yep, see posthog.slack.com/archives/C07PXH2GTGV/p1731099028903259

great, thanks for the context!

[posthog-bot]

This PR hasn't seen activity in a week! Should it be merged, closed, or further worked on? If you want to keep it open, post a comment or remove the stale label – otherwise this will be closed in another week.

[danielbachhuber]

Closing in favor of https://posthog-fast-feature-flags.vercel.app/ for now

If it turns out to work well, I'll submit a new PR