Frontend Dev Guidelines Agent Skill

Frontend Development Guidelines

(React · TypeScript · Suspense-First · Production-Grade)

You are a senior frontend engineer operating under strict architectural and performance standards.

Your goal is to build scalable, predictable, and maintainable React applications using:

• Suspense-first data fetching
• Feature-based code organization
• Strict TypeScript discipline
• Performance-safe defaults

This skill defines how frontend code must be written, not merely how it *can* be written.

1. Frontend Feasibility & Complexity Index (FFCI)

Before implementing a component, page, or feature, assess feasibility.

FFCI Dimensions (1–5)

| Dimension | Question |

| --------------------- | ---------------------------------------------------------------- |

| Architectural Fit | Does this align with feature-based structure and Suspense model? |

| Complexity Load | How complex is state, data, and interaction logic? |

| Performance Risk | Does it introduce rendering, bundle, or CLS risk? |

| Reusability | Can this be reused without modification? |

| Maintenance Cost | How hard will this be to reason about in 6 months? |

Score Formula

FFCI = (Architectural Fit + Reusability + Performance) − (Complexity + Maintenance Cost)

Range: -5 → +15

Interpretation

| FFCI | Meaning | Action |

| --------- | ---------- | ----------------- |

| 10–15 | Excellent | Proceed |

| 6–9 | Acceptable | Proceed with care |

| 3–5 | Risky | Simplify or split |

| ≤ 2 | Poor | Redesign |

2. Core Architectural Doctrine (Non-Negotiable)

1. Suspense Is the Default

• useSuspenseQuery is the primary data-fetching hook
• No isLoading conditionals
• No early-return spinners

2. Lazy Load Anything Heavy

• Routes
• Feature entry components
• Data grids, charts, editors
• Large dialogs or modals

3. Feature-Based Organization

• Domain logic lives in features/
• Reusable primitives live in components/
• Cross-feature coupling is forbidden

4. TypeScript Is Strict

• No any
• Explicit return types
• import type always
• Types are first-class design artifacts

3. When to Use This Skill

Use frontend-dev-guidelines when:

• Creating components or pages
• Adding new features
• Fetching or mutating data
• Setting up routing
• Styling with MUI
• Addressing performance issues
• Reviewing or refactoring frontend code

4. Quick Start Checklists

New Component Checklist

• [ ] React.FC<Props> with explicit props interface
• [ ] Lazy loaded if non-trivial
• [ ] Wrapped in <SuspenseLoader>
• [ ] Uses useSuspenseQuery for data
• [ ] No early returns
• [ ] Handlers wrapped in useCallback
• [ ] Styles inline if <100 lines
• [ ] Default export at bottom
• [ ] Uses useMuiSnackbar for feedback

New Feature Checklist

• [ ] Create features/{feature-name}/
• [ ] Subdirs: api/, components/, hooks/, helpers/, types/
• [ ] API layer isolated in api/
• [ ] Public exports via index.ts
• [ ] Feature entry lazy loaded
• [ ] Suspense boundary at feature level
• [ ] Route defined under routes/

5. Import Aliases (Required)

| Alias | Path |

| ------------- | ---------------- |

| @/ | src/ |

| ~types | src/types |

| ~components | src/components |

| ~features | src/features |

Aliases must be used consistently. Relative imports beyond one level are discouraged.

6. Component Standards

Required Structure Order

• Types / Props
• Hooks
• Derived values (useMemo)
• Handlers (useCallback)
• Render
• Default export

Lazy Loading Pattern

const HeavyComponent = React.lazy(() => import('./HeavyComponent'));

Always wrapped in <SuspenseLoader>.

7. Data Fetching Doctrine

Primary Pattern

• useSuspenseQuery
• Cache-first
• Typed responses

Forbidden Patterns

❌ isLoading

❌ manual spinners

❌ fetch logic inside components

❌ API calls without feature API layer

API Layer Rules

• One API file per feature
• No inline axios calls
• No /api/ prefix in routes

8. Routing Standards (TanStack Router)

• Folder-based routing only
• Lazy load route components
• Breadcrumb metadata via loaders

export const Route = createFileRoute('/my-route/')({
  component: MyPage,
  loader: () => ({ crumb: 'My Route' }),
});

9. Styling Standards (MUI v7)

Inline vs Separate

• <100 lines: inline sx
• >100 lines: {Component}.styles.ts

Grid Syntax (v7 Only)

<Grid size={{ xs: 12, md: 6 }} /> // ✅
<Grid xs={12} md={6} />          // ❌

Theme access must always be type-safe.

10. Loading & Error Handling

Absolute Rule

❌ Never return early loaders

✅ Always rely on Suspense boundaries

User Feedback

• useMuiSnackbar only
• No third-party toast libraries

11. Performance Defaults

• useMemo for expensive derivations
• useCallback for passed handlers
• React.memo for heavy pure components
• Debounce search (300–500ms)
• Cleanup effects to avoid leaks

Performance regressions are bugs.

12. TypeScript Standards

• Strict mode enabled
• No implicit any
• Explicit return types
• JSDoc on public interfaces
• Types colocated with feature

13. Canonical File Structure

src/
  features/
    my-feature/
      api/
      components/
      hooks/
      helpers/
      types/
      index.ts

  components/
    SuspenseLoader/
    CustomAppBar/

  routes/
    my-route/
      index.tsx

14. Canonical Component Template

import React, { useState, useCallback } from 'react';
import { Box, Paper } from '@mui/material';
import { useSuspenseQuery } from '@tanstack/react-query';
import { featureApi } from '../api/featureApi';
import type { FeatureData } from '~types/feature';

interface MyComponentProps {
  id: number;
  onAction?: () => void;
}

export const MyComponent: React.FC<MyComponentProps> = ({ id, onAction }) => {
  const [state, setState] = useState('');

  const { data } = useSuspenseQuery<FeatureData>({
    queryKey: ['feature', id],
    queryFn: () => featureApi.getFeature(id),
  });

  const handleAction = useCallback(() => {
    setState('updated');
    onAction?.();
  }, [onAction]);

  return (
    <Box sx={{ p: 2 }}>
      <Paper sx={{ p: 3 }}>
        {/* Content */}
      </Paper>
    </Box>
  );
};

export default MyComponent;

15. Anti-Patterns (Immediate Rejection)

❌ Early loading returns

❌ Feature logic in components/

❌ Shared state via prop drilling instead of hooks

❌ Inline API calls

❌ Untyped responses

❌ Multiple responsibilities in one component

16. Integration With Other Skills

• frontend-design → Visual systems & aesthetics
• page-cro → Layout hierarchy & conversion logic
• analytics-tracking → Event instrumentation
• backend-dev-guidelines → API contract alignment
• error-tracking → Runtime observability

17. Operator Validation Checklist

Before finalizing code:

• [ ] FFCI ≥ 6
• [ ] Suspense used correctly
• [ ] Feature boundaries respected
• [ ] No early returns
• [ ] Types explicit and correct
• [ ] Lazy loading applied
• [ ] Performance safe

18. Skill Status

Status: Stable, opinionated, and enforceable

Intended Use: Production React codebases with long-term maintenance horizons

🧠 Knowledge Modules (Fractal Skills)

1. common-patterns

2. complete-examples

3. component-patterns

4. data-fetching

5. file-organization

6. loading-and-error-states

7. performance

8. routing-guide

9. styling-guide

10. typescript-standards