From 2efb3f263caa50b9ddfa58fb9188c85a0719d4e9 Mon Sep 17 00:00:00 2001
From: Arjan Zuidema <flecks@gmail.com>
Date: Sat, 16 Jul 2022 01:34:21 +0200
Subject: [PATCH] Check for out of bound reads (#523)

* Check for out of bound reads

* Added changelog entry

* Do an early return in seperator for the last index

* Changed order in if statement for readability
This also gets rid of the dataLength variable

* Removed unnecessary check

Co-authored-by: Talha Naqvi <naqvitalha@gmail.com>
---
 CHANGELOG.md      |  2 ++
 src/FlashList.tsx | 13 ++++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0098f8250..db401fb10 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+- Fixed out of bound read from data
+  - https://github.com/Shopify/flash-list/pull/523
 
 - Added JS only fallbacks for unsupported platforms
   - https://github.com/Shopify/flash-list/pull/518
diff --git a/src/FlashList.tsx b/src/FlashList.tsx
index f364e803f..d047b3da3 100644
--- a/src/FlashList.tsx
+++ b/src/FlashList.tsx
@@ -543,11 +543,18 @@ class FlashList<T> extends React.PureComponent<
   }
 
   private separator = (index: number) => {
-    const leadingItem = this.props.data?.[index];
-    const trailingItem = this.props.data?.[index + 1];
-    if (trailingItem === undefined) {
+    // Make sure we have data and don't read out of bounds
+    if (
+      this.props.data === null ||
+      this.props.data === undefined ||
+      index + 1 >= this.props.data.length
+    ) {
       return null;
     }
+
+    const leadingItem = this.props.data[index];
+    const trailingItem = this.props.data[index + 1];
+
     const props = {
       leadingItem,
       trailingItem,