react Rules

No random numbers in React

Do not generate non-deterministic values like random IDs during render in React components. This causes hydration errors because the server-rendered HTML will not match what the client generates. Avoid using functions like Math.random(), Date.now(), uuid(), or any other source of randomness directly in your render function or JSX. Instead: - Generate IDs in useEffect hooks - Use stable IDs based on props or state - Use refs to store generated values - Use libraries that support SSR (like uuid with specific configuration) Bad: `jsx function UserCard() { // This will generate different values on server and client const id = user-${Math.random()} return ( <div id={id}> <input aria-labelledby={label-${Math.floor(Math.random() * 1000)}} /> </div> ) } ` Good: `jsx function UserCard({ userId }) { // Using stable props for IDs const id = user-${userId} // For dynamic IDs, use useEffect and useState const [randomId, setRandomId] = useState(null) useEffect(() => { // Generate random values after mounting setRandomId(label-${Math.floor(Math.random() * 1000)}) }, []) return <div id={id}>{randomId && <input aria-labelledby={randomId} />}</div> } `

No unused components

Do not leave commented-out components. Either remove unused components entirely or implement them properly. Bad: `jsx function MyComponent() { return ( <div> <Header /> {/* <Sidebar> <Navigation /> </Sidebar> */} <MainContent /> </div> ) } ` Good: `jsx function MyComponent() { return ( <div> <Header /> <MainContent /> </div> ) } `

Typescript DRY

Avoid duplicating code in TypeScript. Extract repeated logic into reusable functions, types, or constants. You may have to search the codebase to see if the method or type is already defined. Bad: `typescript // Duplicated type definitions interface User { id: string name: string } interface UserProfile { id: string name: string } // Magic numbers repeated const pageSize = 10 const itemsPerPage = 10 ` Good: `typescript // Reusable type and constant type User = { id: string name: string } const PAGE_SIZE = 10 `