From 9df383670d1fb4b4a7ec5ee7b6d01cf05df9e9e5 Mon Sep 17 00:00:00 2001
From: Rafa Audibert <rafaeelaudibert@gmail.com>
Date: Wed, 11 Dec 2024 18:24:19 -0300
Subject: [PATCH 1/2] feat: Allow users to customize `fetch` behavior

NextJS SSR v15.1+ will not cache `fetch` requests by default, but some users might want that, specially in development mode.
We're now allowing them to customize our `fetch` requests by setting the new `fetch_options` setting when calling `posthog.init`

This needs to be used carefully, but it should be extremely powerful and useful for NextJS users. We'll add documentation about it on `posthog.com` soon.
---
 src/posthog-core.ts |  4 ++++
 src/request.ts      |  1 +
 src/types.ts        | 16 ++++++++++++++++
 3 files changed, 21 insertions(+)

diff --git a/src/posthog-core.ts b/src/posthog-core.ts
index a310f84e5..3fb2790e2 100644
--- a/src/posthog-core.ts
+++ b/src/posthog-core.ts
@@ -663,6 +663,10 @@ export class PostHog {
         }
         options.compression = options.compression === 'best-available' ? this.compression : options.compression
 
+        // Specially useful if you're doing SSR with NextJS
+        // Users must be careful when tweaking `cache` because they might get out-of-date feature flags
+        options.fetchOptions = options.fetchOptions || this.config.fetch_options
+
         request({
             ...options,
             callback: (response) => {
diff --git a/src/request.ts b/src/request.ts
index 3d5a46bb7..ea4ab1343 100644
--- a/src/request.ts
+++ b/src/request.ts
@@ -165,6 +165,7 @@ const _fetch = (options: RequestOptions) => {
         keepalive: options.method === 'POST' && (estimatedSize || 0) < KEEP_ALIVE_THRESHOLD,
         body,
         signal: aborter?.signal,
+        ...options.fetchOptions,
     })
         .then((response) => {
             return response.text().then((responseText) => {
diff --git a/src/types.ts b/src/types.ts
index 303b501b1..3431b4cde 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -333,6 +333,15 @@ export interface PostHogConfig {
         events_burst_limit?: number
     }
 
+    /** Used when sending data via `fetch`, use with care, this is intentionally meant to be used with NextJS `fetch`
+     *  Incorrect usage may cause out-of-date data for feature flags, actions tracking, etc.
+     *  See https://nextjs.org/docs/app/api-reference/functions/fetch#fetchurl-options
+     */
+    fetch_options?: {
+        cache?: RequestInit['cache']
+        next_options?: NextOptions
+    }
+
     /**
      * PREVIEW - MAY CHANGE WITHOUT WARNING - DO NOT USE IN PRODUCTION
      * whether to wrap fetch and add tracing headers to the request
@@ -440,6 +449,9 @@ export interface RequestResponse {
 
 export type RequestCallback = (response: RequestResponse) => void
 
+// See https://nextjs.org/docs/app/api-reference/functions/fetch#fetchurl-options
+type NextOptions = { revalidate: false | 0 | number; tags: string[] }
+
 export interface RequestOptions {
     url: string
     // Data can be a single object or an array of objects when batched
@@ -452,6 +464,10 @@ export interface RequestOptions {
     timeout?: number
     noRetries?: boolean
     compression?: Compression | 'best-available'
+    fetchOptions?: {
+        cache?: RequestInit['cache']
+        next?: NextOptions
+    }
 }
 
 // Queued request types - the same as a request but with additional queueing information

From f088b432716f4699d1d02359a191fbf9f6380066 Mon Sep 17 00:00:00 2001
From: Rafa Audibert <rafaeelaudibert@gmail.com>
Date: Wed, 11 Dec 2024 18:50:59 -0300
Subject: [PATCH 2/2] test: Add unit tests for new fetch behavior

---
 src/__tests__/request.test.ts | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/__tests__/request.test.ts b/src/__tests__/request.test.ts
index 6456321d0..b098a1d5f 100644
--- a/src/__tests__/request.test.ts
+++ b/src/__tests__/request.test.ts
@@ -186,6 +186,23 @@ describe('request', () => {
             })
         })
 
+        it('supports nextOptions parameter', async () => {
+            request(
+                createRequest({
+                    fetchOptions: { cache: 'force-cache', next: { revalidate: 0, tags: ['test'] } },
+                })
+            )
+            await flushPromises()
+
+            expect(mockedFetch).toHaveBeenCalledWith(
+                expect.anything(),
+                expect.objectContaining({
+                    cache: 'force-cache',
+                    next: { revalidate: 0, tags: ['test'] },
+                })
+            )
+        })
+
         describe('keepalive with fetch and large bodies can cause some browsers to reject network calls', () => {
             it.each([
                 ['always keepalive with small json POST', 'POST', 'small', undefined, true, ''],