chore: add nullish check, early return, and function declaration patterns to AI configs

Author: kimyouknow

.cursorrules

@@ -19,6 +19,10 @@ Mobile depends on core. Core must NOT depend on mobile.
 - Strict boolean checks: `value !== undefined` not `if (value)`
 - Zero runtime dependencies
 - Always return cleanup in useEffect to remove listeners
+- Nullish checks: `== null` for both null and undefined, `!== undefined` only when distinction matters
+- Early returns (guard clauses) over nested if-else blocks
+- Function declarations use `function` keyword, not arrow: `function toggle(state: boolean) { return !state; }`
+- Short inline callbacks (map, filter args) are OK with arrow functions
 
 ## SSR-Safe Pattern
 

.github/copilot-instructions.md

@@ -16,6 +16,19 @@
 - Strict boolean checks: `value !== undefined` not `if (value)`
 - Zero runtime dependencies
 - Always return cleanup in useEffect to remove listeners
+- Nullish checks: use `== null` for both null and undefined:
+  ```ts
+  if (ref == null) { continue; }
+  items.filter(item => item != null);
+  const controlled = valueProp !== undefined; // only when distinction matters
+  ```
+- Prefer early returns (guard clauses) over nested if-else blocks
+- Function declarations use `function` keyword, not arrow functions:
+  ```ts
+  // ✅ function toggle(state: boolean) { return !state; }
+  // ✅ items.filter(item => item != null)  ← inline callback arrow OK
+  // ❌ const toggle = (state: boolean) => !state;
+  ```
 
 ## SSR-Safe Pattern (CRITICAL)
 

AGENTS.md

@@ -48,6 +48,57 @@ src/hooks/useHookName/
 - **No `any` types** — Full TypeScript strict mode
 - **Zero runtime dependencies**
 
+### Nullish Checks and Control Flow
+
+**Use `== null` for nullish checks** — checks both null and undefined:
+
+```ts
+// ✅ Good
+if (ref == null) {
+  continue;
+}
+items.filter(item => item != null);
+
+// ✅ Use !== undefined only when null/undefined distinction matters
+const controlled = valueProp !== undefined;
+```
+
+**Prefer early returns (guard clauses)** over nested if-else:
+
+```ts
+// ✅ Good — guard clause
+function process(value: string | null) {
+  if (value == null) {
+    return DEFAULT;
+  }
+  return transform(value);
+}
+
+// ❌ Bad — nested if-else
+function process(value: string | null) {
+  if (value != null) {
+    return transform(value);
+  } else {
+    return DEFAULT;
+  }
+}
+```
+
+**Function declarations use `function` keyword**, arrow functions only for short inline callbacks:
+
+```ts
+// ✅ Good — function keyword for declarations
+function toggle(state: boolean) {
+  return !state;
+}
+
+// ✅ Good — arrow for inline callbacks
+items.filter(item => item != null);
+
+// ❌ Bad — arrow for function declarations
+const toggle = (state: boolean) => !state;
+```
+
 ### SSR-Safe Pattern
 
 All hooks/utils accessing browser APIs must be SSR-safe: