React Quality

React/TypeScript Code Quality Guidelines

Overview

This skill enforces critical code quality standards for React and TypeScript in the kattle frontend. Apply these rules rigorously when writing, modifying, or reviewing components.

Essential Rules

1. No Type Assertions (as)

Rule: Use explicit type declarations instead of type assertions.

Why: Type assertions bypass TypeScript's type checking and hide potential bugs.

Bad:

const user = data as User;
const count = value as number;

Good:

const user: User = parseUserData(data);
const count: number = parseInt(value, 10);

2. No Unused Imports

Rule: Remove all unused imports from files.

Why: Clutters code, increases bundle size, and creates maintenance confusion.

Check: TypeScript will flag unused imports. Remove them during code review.

3. Safe Array Access

Rule: Always check for undefined before accessing array elements or object properties.

Why: Prevents runtime crashes from accessing undefined values.

Bad:

const item = items[0];
const name = user.profile.name; // what if user or profile is undefined?

Good:

const item = items?.[0];
const name = user?.profile?.name ?? 'Unknown';

4. Explicit Type Annotations for Complex Structures

Rule: Add type annotations for function parameters, return types, and complex state.

Why: Makes code self-documenting and catches type errors early.

Bad:

function processData(data) {
  const result = { ...data };
  return result;
}

const [state, setState] = useState({});

Good:

interface ProcessedData {
  id: string;
  timestamp: Date;
  value: number;
}

function processData(data: unknown): ProcessedData {
  const result: ProcessedData = { ...data as ProcessedData };
  return result;
}

interface AppState {
  user: User | null;
  isLoading: boolean;
}

const [state, setState] = useState<AppState>({
  user: null,
  isLoading: false,
});

5. React Element Keys - Unique and Stable

Rule: Always provide stable, unique keys in lists. Never use array indices.

Why: React uses keys to match elements across renders. Unstable keys cause state loss and bugs.

Bad:

{items.map((item, index) => (
  <div key={index}>{item.name}</div>
))}

Good:

{items.map((item) => (
  <div key={item.id}>{item.name}</div>
))}

6. useCallback for Event Handlers

Rule: Wrap event handler functions in useCallback to prevent unnecessary re-renders of child components.

Why: Prevents children from re-rendering when parent updates, improving performance.

Bad:

const handleClick = () => {
  dispatch(updateState());
};

return <Button />;

Good:

const handleClick = useCallback(() => {
  dispatch(updateState());
}, [dispatch]);

return <Button />;

7. Functional State Updates

Rule: Use the function form of setState when the new state depends on previous state.

Why: Ensures you're always working with the latest state, preventing race conditions.

Bad:

const [count, setCount] = useState(0);
const increment = () => setCount(count + 1);

Good:

const [count, setCount] = useState(0);
const increment = useCallback(() => {
  setCount((prev) => prev + 1);
}, []);

8. Complete Dependency Arrays

Rule: Always include all external values used in hooks (useEffect, useCallback, useMemo) in their dependency arrays.

Why: Missing dependencies cause stale closures and bugs that are hard to debug.

Bad:

useEffect(() => {
  console.log(userId); // userId is missing from deps!
  fetchUser();
}, []);

Good:

useEffect(() => {
  console.log(userId);
  fetchUser();
}, [userId]);

9. Component Decomposition

Rule: Split components when they exceed 300 lines. Extract sections into separate components.

Why: Large components are hard to understand, test, and maintain.

Indicators of a "fat" component:

• Multiple independent sections with their own state
• Deeply nested JSX
• Multiple responsibilities (data fetching, UI rendering, business logic)

Action: Extract into smaller, focused components with clear interfaces.

10. Props and State Typing

Rule: Define explicit interfaces for component props. Never use any or unknown without narrowing.

Why: Makes component contracts clear and catches prop errors early.

Bad:

function Card(props: any) {
  return <div>{props.content}</div>;
}

Good:

interface CardProps {
  content: string;
  variant?: 'default' | 'outlined';
  onClick?: (id: string) => void;
}

function Card({ content, variant = 'default', onClick }: CardProps) {
  return <div => onClick?.(content)}>{content}</div>;
}

Quick Checklist

Before submitting code or approving a PR, verify:

• No as type assertions
• No unused imports
• All array/object access is safe (using optional chaining or checks)
• Function parameters and return types have explicit annotations
• Component props defined in a clear interface
• List items have stable, unique key props
• Event handlers wrapped in useCallback (if passed to children)
• State updates use functional form when dependent on previous state
• All hook dependencies are complete and correct
• Components are decomposed (max 300 lines)
• No any types without narrowing

When to Apply This Skill

Apply these rules when:

• Writing new React components
• Reviewing pull requests with TypeScript/React changes
• Refactoring existing components
• Debugging type-related issues

Prioritize rules 1-8 as they prevent the most common and serious bugs.

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