initial commit

.agents/skills/vercel-react-native-skills/rules/list-performance-inline-objects.md

@@ -0,0 +1,97 @@
+---
+title: Avoid Inline Objects in renderItem
+impact: HIGH
+impactDescription: prevents unnecessary re-renders of memoized list items
+tags: lists, performance, flatlist, virtualization, memo
+---
+
+## Avoid Inline Objects in renderItem
+
+Don't create new objects inside `renderItem` to pass as props. Inline objects
+create new references on every render, breaking memoization. Pass primitive
+values directly from `item` instead.
+
+**Incorrect (inline object breaks memoization):**
+
+```tsx
+function UserList({ users }: { users: User[] }) {
+  return (
+    <LegendList
+      data={users}
+      renderItem={({ item }) => (
+        <UserRow
+          // Bad: new object on every render
+          user={{ id: item.id, name: item.name, avatar: item.avatar }}
+        />
+      )}
+    />
+  );
+}
+```
+
+**Incorrect (inline style object):**
+
+```tsx
+renderItem={({ item }) => (
+  <UserRow
+    name={item.name}
+    // Bad: new style object on every render
+    style={{ backgroundColor: item.isActive ? 'green' : 'gray' }}
+  />
+)}
+```
+
+**Correct (pass item directly or primitives):**
+
+```tsx
+function UserList({ users }: { users: User[] }) {
+  return (
+    <LegendList
+      data={users}
+      renderItem={({ item }) => (
+        // Good: pass the item directly
+        <UserRow user={item} />
+      )}
+    />
+  );
+}
+```
+
+**Correct (pass primitives, derive inside child):**
+
+```tsx
+renderItem={({ item }) => (
+  <UserRow
+    id={item.id}
+    name={item.name}
+    isActive={item.isActive}
+  />
+)}
+
+const UserRow = memo(function UserRow({ id, name, isActive }: Props) {
+  // Good: derive style inside memoized component
+  const backgroundColor = isActive ? 'green' : 'gray'
+  return <View style={[styles.row, { backgroundColor }]}>{/* ... */}</View>
+})
+```
+
+**Correct (hoist static styles in module scope):**
+
+```tsx
+const activeStyle = { backgroundColor: 'green' }
+const inactiveStyle = { backgroundColor: 'gray' }
+
+renderItem={({ item }) => (
+  <UserRow
+    name={item.name}
+    // Good: stable references
+    style={item.isActive ? activeStyle : inactiveStyle}
+  />
+)}
+```
+
+Passing primitives or stable references allows `memo()` to skip re-renders when
+the actual values haven't changed.
+
+**Note:** If you have the React Compiler enabled, it handles memoization
+automatically and these manual optimizations become less critical.