Add PostHog feature flag example

edge-functions/feature-flag-posthog/.env.example

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+# This is the PostHog Project API Key
+NEXT_PUBLIC_POSTHOG_PROJECT_API_KEY=
+
+# This is the PostHog host.
+# For PostHog cloud this would be https://app.posthog.com
+NEXT_PUBLIC_POSTHOG_HOST=

edge-functions/feature-flag-posthog/.gitignore

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+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+/.pnp
+.pnp.js
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# local env files
+.env.local
+.env.development.local
+.env.test.local
+.env.production.local
+
+# vercel
+.vercel

edge-functions/feature-flag-posthog/README.md

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+# A/B Testing with PostHog
+
+[PostHog](https://posthog.com/) is an open-source product analytics platform that offers a suite of tools, including funnels, heat maps, session recording, feature flags and more, all in a single platform.
+
+## Demo
+
+https://edge-functions-feature-flag-posthog.vercel.sh
+
+### One-Click Deploy
+
+Deploy the example using [Vercel](https://vercel.com?utm_source=github&utm_medium=readme):
+
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/new/clone?repository-url=https://github.com/vercel/examples/tree/main/edge-functions/feature-flag-posthog&env=NEXT_PUBLIC_POSTHOG_PROJECT_API_KEY,NEXT_PUBLIC_POSTHOG_HOST&project-name=feature-flag-posthog&repository-name=feature-flag-posthog)
+
+## Getting Started
+
+Execute [`create-next-app`](https://github.com/vercel/next.js/tree/canary/packages/create-next-app) with [npm](https://docs.npmjs.com/cli/init) or [Yarn](https://yarnpkg.com/lang/en/docs/cli/create/) to bootstrap the example:
+
+```bash
+npx create-next-app --example https://github.com/vercel/examples/tree/main/edge-functions/feature-flag-posthog feature-flag-posthog
+# or
+yarn create next-app --example https://github.com/vercel/examples/tree/main/edge-functions/feature-flag-posthog feature-flag-posthog
+```
+
+You'll need to have an account with [PostHog](https://posthog.com/signup). Once that's done, copy the `.env.example` file in this directory to `.env.local` (which will be ignored by Git):
+
+```bash
+cp .env.example .env.local
+```
+
+Then open `.env.local` and set the environment variables to match the host for your PostHog instance and the Project API key available under Project settings - Project API Key.
+
+Next, run Next.js in development mode:
+
+```bash
+npm install
+npm run dev
+
+# or
+
+yarn
+yarn dev
+```

edge-functions/feature-flag-posthog/lib/constants.ts

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+/**
+ * List of known active Feature Flags
+ */
+export const FEATURE_FLAGS = {
+  NEW_ABOUT_PAGE: 'New_About_Page',
+  NEW_MARKETING_PAGE: 'New_Marketing_Page',
+  NEW_PRODUCT_PAGE: 'New_Product_Page',
+} as const
+
+export type FEATURE_FLAGS = typeof FEATURE_FLAGS[keyof typeof FEATURE_FLAGS]
+
+export const DISTINCT_ID_COOKIE_NAME = 'distinct_id'

edge-functions/feature-flag-posthog/lib/posthog-node.ts

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+import { FEATURE_FLAGS } from './constants'
+
+/**
+ * Checks if a feature flag is enabled.
+ *
+ * @param distinctUserId A unique identifier for the user
+ * @param featureName The name of the feature flag
+ * @returns true if the feature flag is enabled. Otherwise, false.
+ */
+export async function isFeatureFlagEnabled(
+  distinctUserId: string,
+  featureName: FEATURE_FLAGS
+): Promise<boolean> {
+  console.log('isFeatureEnabled', distinctUserId, featureName)
+
+  const featureFlagValue = await getFeatureFlagVariant(
+    distinctUserId,
+    featureName
+  )
+
+  const featureEnabled = featureFlagValue ? true : false
+  console.log('featureEnabled', featureEnabled)
+
+  return featureEnabled
+}
+
+/**
+ * Retrieves the value of the feature flag.
+ *
+ * @param distinctUserId A unique identifier for the user
+ * @param featureName The name of the feature flag
+ * @returns If the feature flag is an A/B test, then the value may be true or undefined.
+ *          If the feature flag is a multvariate, then the value will be a string
+ */
+export async function getFeatureFlagVariant(
+  distinctUserId: string,
+  featureName: FEATURE_FLAGS
+): Promise<string | boolean | undefined> {
+  console.log('getFeatureFlagVariant', distinctUserId, featureName)
+
+  const res = await fetch(
+    `${process.env.NEXT_PUBLIC_POSTHOG_HOST}/decide?v=2`,
+    {
+      method: 'POST',
+      body: JSON.stringify({
+        api_key: process.env.NEXT_PUBLIC_POSTHOG_PROJECT_API_KEY,
+        distinct_id: distinctUserId,
+      }),
+    }
+  )
+
+  if (!res.ok) {
+    throw new Error(
+      `Fetch request to retrieve the ${featureName} flag status failed with: (${res.status}) ${res.statusText}`
+    )
+  }
+
+  const data = await res.json()
+  console.log(data)
+
+  return data.featureFlags[featureName]
+}

edge-functions/feature-flag-posthog/lib/posthog.ts

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+import { useEffect } from 'react'
+import { useRouter } from 'next/router'
+import posthog, { PostHog } from 'posthog-js'
+
+let POSTHOG_INSTANCE: PostHog
+
+export const usePostHog = (
+  apiKey: string,
+  config?: posthog.Config,
+  name?: string
+): void => {
+  const router = useRouter()
+
+  if (config.loaded) {
+    // override the existing loaded function so we can store a reference
+    // to the PostHog instance
+    const oldLoaded = config.loaded
+    config.loaded = (posthog: PostHog) => {
+      setPostHogInstance(posthog)
+      oldLoaded(POSTHOG_INSTANCE)
+    }
+  } else {
+    config.loaded = setPostHogInstance
+  }
+
+  useEffect((): (() => void) => {
+    if (typeof window === undefined) return
+
+    posthog.init(apiKey, config, name)
+
+    // Track page views
+    const handleRouteChange = () => posthog.capture('$pageview')
+    router.events.on('routeChangeComplete', handleRouteChange)
+
+    return () => {
+      router.events.off('routeChangeComplete', handleRouteChange)
+    }
+  }, [router.events])
+}
+
+const setPostHogInstance = (posthog: PostHog) => {
+  POSTHOG_INSTANCE = posthog
+}
+
+export const getPostHogInstance = (): PostHog => {
+  return POSTHOG_INSTANCE
+}

edge-functions/feature-flag-posthog/next-env.d.ts

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+/// <reference types="next" />
+/// <reference types="next/types/global" />
+/// <reference types="next/image-types/global" />
+
+// NOTE: This file should not be edited
+// see https://nextjs.org/docs/basic-features/typescript for more information.

edge-functions/feature-flag-posthog/next.config.js

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+const withTM = require('@vercel/edge-functions-ui/transpile')()
+
+module.exports = withTM()