fix(browser): skip deep copy for snapshot events to prevent stack overflow (#3298)

* fix(browser): skip deep copy for snapshot events to prevent stack overflow

$snapshot and $exception events set _noTruncate which means no string
truncation is needed. Move the guard to the call site so we skip
_copyAndTruncateStrings entirely, and tighten the parameter type from
number | null to number. This avoids stack overflows on deeply nested
rrweb DOM snapshots from complex pages.

* chore: add changeset

Author: TueHaulund

.changeset/skip-deep-copy-snapshots.md

@@ -0,0 +1,5 @@
+---
+"posthog-js": patch
+---
+
+fix: skip deep copy for snapshot/exception events to prevent stack overflow on deeply nested DOM trees

packages/browser/src/__tests__/utils.test.ts

@@ -54,10 +54,6 @@ describe('utils', () => {
             expect(copy).not.toEqual(target)
         })
 
-        it('does not truncate when passed null', () => {
-            expect(_copyAndTruncateStrings(target, null)).toEqual(target)
-        })
-
         it('handles recursive objects', () => {
             const recursiveObject: Record<string, any> = { key: 'vaaaaalue', values: ['fooobar'] }
             recursiveObject.values.push(recursiveObject)

packages/browser/src/posthog-core.ts

@@ -1269,7 +1269,9 @@ export class PostHog implements PostHogInterface {
             data.$set_once = setOnceProperties
         }
 
-        data = _copyAndTruncateStrings(data, options?._noTruncate ? null : this.config.properties_string_max_length)
+        if (!options?._noTruncate) {
+            data = _copyAndTruncateStrings(data, this.config.properties_string_max_length)
+        }
         data.timestamp = timestamp
         if (!isUndefined(options?.timestamp)) {
             data.properties['$event_time_override_provided'] = true

packages/browser/src/utils/index.ts

@@ -1,6 +1,6 @@
 import { PostHogConfig, Properties } from '../types'
 import { logger } from './logger'
-import { isFormData, isNull, isNullish, isNumber, isString, hasOwnProperty, isArray } from '@posthog/core'
+import { isFormData, isNullish, isNumber, isString, hasOwnProperty, isArray } from '@posthog/core'
 
 export function find<T>(value: T[], predicate: (value: T) => boolean): T | undefined {
     for (let i = 0; i < value.length; i++) {
@@ -144,10 +144,10 @@ function deepCircularCopy<T extends Record<string, any> = Record<string, any>>(
 
 export function _copyAndTruncateStrings<T extends Record<string, any> = Record<string, any>>(
     object: T,
-    maxStringLength: number | null
+    maxStringLength: number
 ): T {
     return deepCircularCopy(object, (value: any) => {
-        if (isString(value) && !isNull(maxStringLength)) {
+        if (isString(value)) {
             return (value as string).slice(0, maxStringLength)
         }
         return value