Add PostHog analytics

lib/config.ts

@@ -6,8 +6,10 @@
  */
 
 import { parsePageId } from 'notion-utils'
-import { getSiteConfig, getEnv } from './get-config-value'
-import { PageUrlOverridesMap, PageUrlOverridesInverseMap } from './types'
+import posthog from 'posthog-js'
+import { getEnv, getSiteConfig } from './get-config-value'
+import { PageUrlOverridesInverseMap, PageUrlOverridesMap } from './types'
+import Config = posthog.Config
 
 export const rootNotionPageId: string = parsePageId(
   getSiteConfig('rootNotionPageId'),
@@ -119,13 +121,23 @@ export const api = {
 // ----------------------------------------------------------------------------
 
 export const fathomId = isDev ? null : process.env.NEXT_PUBLIC_FATHOM_ID
-
 export const fathomConfig = fathomId
   ? {
       excludedDomains: ['localhost', 'localhost:3000']
     }
   : undefined
 
+// PostHog automatically filters events coming from localhost
+export const postHogId = process.env.NEXT_PUBLIC_POSTHOG_ID
+export const postHogConfig: Config = {
+  // See https://posthog.com/docs/integrate/client/js#config
+  api_host: 'https://app.posthog.com',
+  loaded: (posthog_instance) => {
+    console.debug(`PostHog loaded`, posthog_instance)
+    // posthog_instance.identify(unique user id)
+  }
+}
+
 function cleanPageUrlMap(
   pageUrlMap: PageUrlOverridesMap,
   label: string