Ln 721 Frontend Restructure

---
name: ln-721-frontend-restructure
description: Restructures React frontend from monolith to component-based architecture
---

# ln-721-frontend-restructure

**Type:** L3 Worker
**Category:** 7XX Project Bootstrap
**Parent:** ln-720-structure-migrator

Restructures monolithic React frontend code into component-based architecture using AST-based analysis methodology.

---

## Purpose & Scope

| Aspect | Description |
|--------|-------------|
| **Input** | Monolithic React frontend source |
| **Output** | Component-based architecture with co-located feature folders |
| **Framework** | React only |

**Scope boundaries:**
- Restructures existing code, does not add new functionality
- Works with React + TypeScript projects
- Applies transformation rules from reference files

---

## Workflow

| Phase | Name | Actions | Output |
|-------|------|---------|--------|
| 1 | Analyze | Scan source, detect component types, measure complexity | File inventory, complexity metrics |
| 2 | Plan | Apply split thresholds, calculate file moves, detect conflicts | Migration plan |
| 3 | Execute | Create folders, extract content, update imports | Restructured files |
| 4 | Verify | Run build, check imports, validate structure | Build success report |

---

## Phase 1: Analyze

Scan current frontend structure and classify components.

| Step | Action | Reference |
|------|--------|-----------|
| 1.1 | Scan all `.tsx` and `.ts` files in source | — |
| 1.2 | Measure file complexity (lines, hooks, types) | `transformation_rules.md` |
| 1.3 | Classify components by category | `component_patterns.md` |
| 1.4 | Build import dependency graph | `import_strategy.md` |

**Output:** Component inventory with classifications and metrics.

---

## Phase 2: Plan

Generate migration plan based on analysis.

| Step | Action | Reference |
|------|--------|-----------|
| 2.1 | Apply split thresholds to identify files to restructure | `transformation_rules.md` |
| 2.2 | Calculate target paths for each file | `react_folder_structure.md` |
| 2.3 | Identify import updates needed | `import_strategy.md` |
| 2.4 | Detect potential conflicts (name collisions, circular deps) | — |

**Output:** Migration plan with Before/After mapping.

---

## Phase 3: Execute

Apply transformations in correct order.

| Step | Action | Notes |
|------|--------|-------|
| 3.1 | Create directory structure | All target folders |
| 3.2 | Extract types to `types.ts` | Types have no dependencies |
| 3.3 | Extract constants to `constants.ts` | Constants depend only on types |
| 3.4 | Extract hooks to `hooks.ts` | Hooks depend on types, constants |
| 3.5 | Extract sub-components | Components use all above |
| 3.6 | Create barrel exports (`index.ts`) | For clean imports |
| 3.7 | Update all import paths | Fix references |

**Order is critical:** Execute in sequence to avoid broken imports.

---

## Phase 4: Verify

Validate restructured project.

| Check | Command | Expected |
|-------|---------|----------|
| TypeScript compilation | `npx tsc --noEmit` | No errors |
| Build | `npm run build` | Success |
| No orphan files | Manual check | Source location empty |
| Imports resolve | Build success | No module not found errors |

---

## Transformation Summary

| Transformation | Before State | After State |
|----------------|--------------|-------------|
| Component Split | Single file >300 lines | Feature folder with co-located files |
| Type Extraction | Inline interfaces | Separate `types.ts` |
| Constant Extraction | Inline magic values | Separate `constants.ts` |
| Hook Extraction | Inline useState/useEffect | Separate `hooks.ts` or shared hooks |
| UI Component Move | Scattered in features | Centralized in `components/ui/` |
| Layout Component Move | Mixed with features | Centralized in `components/layout/` |

---

## Critical Rules

- **Single Responsibility:** Handle only frontend restructuring, no backend changes
- **Idempotent:** Can re-run without duplicate files or corruption
- **Build Verification:** Must verify `npm run build` passes after changes
- **Preserve Functionality:** No behavioral changes, only structural
- **Backup Strategy:** Do not delete source files until verification passes
- **Import Consistency:** Use path aliases for shared, relative for co-located

---

## Definition of Done

- [ ] All source files analyzed and classified
- [ ] Migration plan generated with Before/After mapping
- [ ] Directory structure created per template
- [ ] All extractions completed (types, constants, hooks, components)
- [ ] Import paths updated throughout project
- [ ] `npm run build` passes successfully
- [ ] No orphan imports or missing files
- [ ] Barrel exports created for shared folders

---

## Risk Mitigation

| Risk | Detection | Mitigation |
|------|-----------|------------|
| Build failure after restructure | `npm run build` fails | Rollback: restore from source, investigate specific error |
| Missing imports | Module not found errors | Scan all imports before/after, update missed paths |
| Circular dependencies | Build warning or runtime error | Analyze dependency graph, break cycles by extracting shared code |
| Lost functionality | Tests fail or UI broken | Run existing tests before/after transformation |
| Name collisions | Duplicate export names | Rename with feature prefix before moving |

---

## Reference Files

| File | Purpose |
|------|---------|
| `references/transformation_rules.md` | Split thresholds, extraction rules, transformation order |
| `references/component_patterns.md` | Component classification by category |
| `references/import_strategy.md` | Import update rules, path aliases, barrel exports |
| `references/react_folder_structure.md` | Target directory structure template |

---

**Version:** 2.0.0
**Last Updated:** 2026-01-10