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

@@ -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

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,