Add support for using PH on the NextJS server side

front_end/src/utils/posthog.server.ts

@@ -0,0 +1,48 @@
+import "server-only";
+
+import { cookies } from "next/headers";
+import { PostHog } from "posthog-node";
+
+import { getPublicSettings } from "./public_settings.server";
+
+let posthogClient: PostHog | null = null;
+
+function getPostHogClient(): PostHog | null {
+  const { PUBLIC_POSTHOG_KEY, PUBLIC_POSTHOG_BASE_URL } = getPublicSettings();
+  const apiKey = PUBLIC_POSTHOG_KEY;
+  if (!apiKey) return null;
+
+  if (!posthogClient) {
+    posthogClient = new PostHog(apiKey, {
+      host: PUBLIC_POSTHOG_BASE_URL || "https://us.i.posthog.com",
+      flushAt: 1,
+      flushInterval: 0,
+    });
+  }
+  return posthogClient;
+}
+
+export async function getFeatureFlag(
+  flagName: string,
+  defaultValue: boolean | string = false
+): Promise<boolean | string> {
+  const client = getPostHogClient();
+  if (!client) return defaultValue;
+
+  const { PUBLIC_POSTHOG_KEY } = getPublicSettings();
+
+  const cookieStore = await cookies();
+  const cookieName = "ph_" + PUBLIC_POSTHOG_KEY + "_posthog";
+  const cookieValue = cookieStore.get(cookieName)?.value;
+  const distinctId = cookieValue
+    ? JSON.parse(cookieValue).distinct_id
+    : "anonymous";
+
+  try {
+    const flag = await client.getFeatureFlag(flagName, distinctId);
+
+    return flag !== undefined ? flag : defaultValue;
+  } catch {
+    return defaultValue;
+  }
+}