fix: Eager instantiation of featureFlags

This closes a behavioral gap where `.featureFlags` would return a stub
before `init()` is called

Author: dustinbyrne

packages/browser/src/__tests__/extension-classes.test.ts

@@ -88,6 +88,7 @@ describe('__extensionClasses enrollment', () => {
 
         const posthog = new PostHog()
 
+        expect(posthog._featureFlags).toBeDefined()
         expect(posthog.toolbar).toBeDefined()
         expect(posthog.surveys).toBeDefined()
         expect(posthog.conversations).toBeDefined()
@@ -101,6 +102,7 @@ describe('__extensionClasses enrollment', () => {
 
         const posthog = new PostHog()
 
+        expect(posthog._featureFlags).toBeUndefined()
         expect(posthog.toolbar).toBeUndefined()
         expect(posthog.surveys).toBeUndefined()
         expect(posthog.conversations).toBeUndefined()

packages/browser/src/posthog-core.ts

@@ -442,6 +442,7 @@ export class PostHog implements PostHogInterface {
         // Eagerly construct extensions from default classes so they're available before init().
         // For the slim bundle, these remain undefined until _initExtensions sets them from config.
         const ext = PostHog.__defaultExtensionClasses ?? {}
+        this._featureFlags = ext.featureFlags && new ext.featureFlags(this)
         this.toolbar = ext.toolbar && new ext.toolbar(this)
         this.surveys = ext.surveys && new ext.surveys(this)
         this.conversations = ext.conversations && new ext.conversations(this)
@@ -707,7 +708,7 @@ export class PostHog implements PostHogInterface {
         // Due to name mangling, we can't easily iterate and assign these extensions
         // The assignment needs to also be mangled. Thus, the loop is unrolled.
         if (ext.featureFlags) {
-            this._extensions.push((this._featureFlags = new ext.featureFlags(this)))
+            this._extensions.push((this._featureFlags = this.featureFlags ?? new ext.featureFlags(this)))
         }
         if (ext.exceptions) {
             this._extensions.push((this.exceptions = this.exceptions ?? new ext.exceptions(this)))