Frontend fundamentals

Frontend Fundamentals Review

"A component should do ONE thing well. If you're describing it with 'and', split it."

When to Apply

Activate this skill when reviewing:

• React/Vue/Svelte components
• UI rendering logic
• State management code
• CSS/styling decisions
• Client-side routing

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Review Checklist

Component Architecture

• Single Responsibility: Does each component do ONE job?
• Size Check: Is the component under 200 lines?
• Props Count: Are there fewer than 7 props?
• Naming: Can you describe the component without saying "and"?

State Management

• Colocation: Is state as close as possible to where it's used?
• Lifting: Is state shared properly between siblings via parent?
• Context vs Props: Is prop drilling avoided (max 3 levels)?
• Server State: Is server data managed separately (React Query/SWR)?

Performance

• Memoization: Are expensive computations wrapped in useMemo?
• Callbacks: Are event handlers wrapped in useCallback where needed?
• Re-renders: Will this cause unnecessary re-renders?
• Lazy Loading: Are heavy components code-split?

Accessibility

• Semantic HTML: Are proper elements used (button vs div)?
• ARIA: Are interactive elements accessible?
• Keyboard: Can users navigate without a mouse?

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Common Mistakes (Anti-Patterns)

1. God Components

❌ UserDashboard.tsx (1000 lines)
   - fetches data, manages state, renders UI, handles routing

✅ Split into:
   - UserDashboardPage.tsx (container)
   - UserStats.tsx (presentation)
   - UserActivity.tsx (presentation)
   - useUserData.ts (hook)

2. Logic in Render

❌ return <div>{users.filter(u => u.active).map(u => ...)}</div>

✅ const activeUsers = useMemo(() => users.filter(u => u.active), [users]);
   return <div>{activeUsers.map(u => ...)}</div>

3. Prop Drilling

❌ <App user={user}>
     <Layout user={user}>
       <Main user={user}>
         <Widget user={user} />

✅ const user = useUser(); // in Widget.tsx

4. Boolean Prop Soup

❌ <Button primary secondary large small disabled loading />

✅ <Button variant="primary" size="large" state="loading" />

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Socratic Questions

Ask the junior these questions instead of giving answers:

1. Architecture: "What is the ONE job of this component?"
2. Splitting: "If I asked you to use just the header part elsewhere, could you?"
3. State: "Who needs this data? Should it live here or higher up?"
4. Performance: "What happens when the parent re-renders?"
5. Complexity: "Could a new developer understand this in 5 minutes?"

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Standards Reference

See detailed patterns in:

• /standards/frontend/component-architecture.md

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Red Flags to Call Out

Flag	Question to Ask
File > 200 lines	"Can we split this into smaller pieces?"
> 5 useState calls	"Should some of this state be lifted or combined?"
useEffect with [] deps but uses external values	"Are we missing dependencies?"
Direct DOM manipulation	"Is there a React way to do this?"
Inline styles everywhere	"Should we use a consistent styling approach?"