feat: enhance TypeScript coding style guidelines with detailed examples and best practices esp interfaces and types

rules/typescript/coding-style.md

@@ -9,19 +9,128 @@ paths:
 
 > This file extends [common/coding-style.md](../common/coding-style.md) with TypeScript/JavaScript specific content.
 
+## Types and Interfaces
+
+Use types to make public APIs, shared models, and component props explicit, readable, and reusable.
+
+### Public APIs
+
+- Add parameter and return types to exported functions, shared utilities, and public class methods
+- Let TypeScript infer obvious local variable types
+- Extract repeated inline object shapes into named types or interfaces
+
+```typescript
+// WRONG: Exported function without explicit types
+export function formatUser(user) {
+  return `${user.firstName} ${user.lastName}`
+}
+
+// CORRECT: Explicit types on public APIs
+interface User {
+  firstName: string
+  lastName: string
+}
+
+export function formatUser(user: User): string {
+  return `${user.firstName} ${user.lastName}`
+}
+```
+
+### Interfaces vs. Type Aliases
+
+- Use `interface` for object shapes that may be extended or implemented
+- Use `type` for unions, intersections, tuples, mapped types, and utility types
+- Prefer string literal unions over `enum` unless an `enum` is required for interoperability
+
+```typescript
+interface User {
+  id: string
+  email: string
+}
+
+type UserRole = 'admin' | 'member'
+type UserWithRole = User & {
+  role: UserRole
+}
+```
+
+### Avoid `any`
+
+- Avoid `any` in application code
+- Use `unknown` for external or untrusted input, then narrow it safely
+- Use generics when a value's type depends on the caller
+
+```typescript
+// WRONG: any removes type safety
+function getErrorMessage(error: any) {
+  return error.message
+}
+
+// CORRECT: unknown forces safe narrowing
+function getErrorMessage(error: unknown): string {
+  if (error instanceof Error) {
+    return error.message
+  }
+
+  return 'Unknown error'
+}
+```
+
+### React Props
+
+- Define component props with a named `interface` or `type`
+- Type callback props explicitly
+- Do not use `React.FC` unless there is a specific reason to do so
+
+```typescript
+interface User {
+  id: string
+  email: string
+}
+
+interface UserCardProps {
+  user: User
+  onSelect: (id: string) => void
+}
+
+function UserCard({ user, onSelect }: UserCardProps) {
+  return <button onClick={() => onSelect(user.id)}>{user.email}</button>
+}
+```
+
+### JavaScript Files
+
+- In `.js` and `.jsx` files, use JSDoc when types improve clarity and a TypeScript migration is not practical
+- Keep JSDoc aligned with runtime behavior
+
+```javascript
+/**
+ * @param {{ firstName: string, lastName: string }} user
+ * @returns {string}
+ */
+export function formatUser(user) {
+  return `${user.firstName} ${user.lastName}`
+}
+```
+
 ## Immutability
 
 Use spread operator for immutable updates:
 
 ```typescript
+interface User {
+  id: string
+  name: string
+}
+
 // WRONG: Mutation
-function updateUser(user, name) {
-  user.name = name  // MUTATION!
+function updateUser(user: User, name: string): User {
+  user.name = name // MUTATION!
   return user
 }
 
 // CORRECT: Immutability
-function updateUser(user, name) {
+function updateUser(user: Readonly<User>, name: string): User {
   return {
     ...user,
     name
@@ -31,31 +140,50 @@ function updateUser(user, name) {
 
 ## Error Handling
 
-Use async/await with try-catch:
+Use async/await with try-catch and narrow unknown errors safely:
 
 ```typescript
-try {
-  const result = await riskyOperation()
-  return result
-} catch (error) {
-  console.error('Operation failed:', error)
-  throw new Error('Detailed user-friendly message')
+interface User {
+  id: string
+  email: string
+}
+
+declare function riskyOperation(userId: string): Promise<User>
+
+function getErrorMessage(error: unknown): string {
+  if (error instanceof Error) {
+    return error.message
+  }
+
+  return 'Unexpected error'
+}
+
+async function loadUser(userId: string): Promise<User> {
+  try {
+    const result = await riskyOperation(userId)
+    return result
+  } catch (error: unknown) {
+    console.error('Operation failed:', error)
+    throw new Error(getErrorMessage(error))
+  }
 }
 ```
 
 ## Input Validation
 
-Use Zod for schema-based validation:
+Use Zod for schema-based validation and infer types from the schema:
 
 ```typescript
 import { z } from 'zod'
 
-const schema = z.object({
+const userSchema = z.object({
   email: z.string().email(),
   age: z.number().int().min(0).max(150)
 })
 
-const validated = schema.parse(input)
+type UserInput = z.infer<typeof userSchema>
+
+const validated: UserInput = userSchema.parse(input)
 ```
 
 ## Console.log