React Modernization Analyzer

"The best time to modernize was five years ago. The second best time is now." -- Adapted from software engineering practice

"Big bang rewrites fail. Incremental migration succeeds." -- Martin Fowler, Refactoring

Core Philosophy

Legacy React codebases accumulate technical debt in predictable patterns: class components with lifecycle methods that hooks express more clearly, Create React App's unmaintained and slow build, React versions behind on concurrent features and security patches, untyped JavaScript where TypeScript would catch whole bug classes, Enzyme tests coupled to implementation, and god-object Redux stores that predate Redux Toolkit.

This skill assesses, quantifies, and plans — it does NOT perform the migration. The output is a prioritized modernization plan with risk scores, effort estimates, and a recommended sequence. The plan is the deliverable; execution is a separate task (often via react-feature-slice / react-app-scaffolder).

Grounding note: the KB has no React corpus. Use collection="javascript" for the JS→TS path and cite react.dev (upgrade guides) + Vite/RTL docs for version-specific detail. Never invent a react collection.

Non-Negotiable Constraints:

1. Assess before acting — never recommend a migration path without evidence from the codebase
2. Incremental over big-bang — every recommendation must be achievable in phases, not a single rewrite
3. Preserve behavior unchanged — modernization, not reimplementation; the UI behaves identically after
4. Dependencies are the real blockers — a React 19 upgrade blocked by an unmaintained UI library is not viable
5. Every recommendation must cite evidence — "this looks like class components" is not evidence; grep -rn "extends React.Component" is

What this skill is NOT: not a migration execution tool (plan, not code); not a code quality review (use react-architecture-checklist); not a security review (use react-security-review).

The ten domain principles and the grounded-KB lookup table live in references/domain-principles.md.

Workflow

Phase 1: SCAN

Objective: Inventory the codebase to understand its current state.

# React + tooling versions
grep -E '"(react|react-dom|react-scripts|typescript|vite|enzyme|redux|@reduxjs/toolkit)"' package.json

# Class components (class→hooks surface)
grep -rln "extends React.Component\|extends Component\|extends PureComponent" src/ | wc -l

# JavaScript vs TypeScript surface
find src -name "*.jsx" -o -name "*.js" | wc -l        # JS to migrate
find src -name "*.tsx" -o -name "*.ts" | wc -l        # already TS

# Build tooling
grep -l "react-scripts" package.json && echo "CRA — migrate to Vite"

# Deprecated lifecycles / APIs
grep -rn "componentWillMount\|componentWillReceiveProps\|UNSAFE_\|defaultProps" src/ | head

# React Compiler adoption (stable since Oct 2025 — check before recommending manual memoization)
grep -l "babel-plugin-react-compiler" package.json || echo "Compiler not wired — candidate path"

# Test framework
grep -rln "from 'enzyme'\|require('enzyme')" src/ | wc -l      # Enzyme = React 18 blocker
grep -rln "@testing-library/react" src/ | wc -l

# Ad-hoc data fetching
grep -rn "componentDidMount" src/ | wc -l
grep -rn "useEffect" src/ | grep -c "fetch\|axios"

Phase 2: ASSESS

Objective: Score each migration path by effort, risk, and blocker potential.

Use references/migration-risk-matrix.md for scoring guidance and references/react-version-migration.md for per-version breaking-change detail.

Phase 3: PLAN

Objective: Produce a prioritized, phased modernization plan.

1. Phase 0: Prerequisites — RTL characterization tests, CI, Node/version pinning (if missing)
2. Phase 1: Tooling wins — CRA → Vite; introduce TypeScript with allowJs
3. Phase 2: Test framework — Enzyme → RTL (unblocks the version bump)
4. Phase 3: Version — React 17 → 18 (then → 19), fixing breaking changes
5. Phase 4: Components — class → hooks, slice by slice
6. Phase 5: State & data — legacy Redux → RTK/Zustand; ad-hoc fetch → query cache
7. Phase 6: Types — finish JS → TS; turn on strict
8. Phase 7: Compiler — wire React Compiler once the version bump lands; strip hand-written memo/useMemo/useCallback the compiler now subsumes

Phase 4: REPORT

Objective: Deliver the plan with evidence, risk scores, and effort estimates. Use the template in references/assessment-output-template.md and the risk scoring table in references/migration-risk-matrix.md.

State Block

<react-modernization-state>
  phase: SCAN | ASSESS | PLAN | REPORT | COMPLETE
  react_version: [16 / 17 / 18 / 19 / unknown]
  bundler: cra | vite | webpack-custom | next | unknown
  language: js | mixed | ts
  ts_strict: true | false | n/a
  class_components: [count]
  test_framework: enzyme | rtl | mixed | none
  state_lib: legacy-redux | rtk | zustand | context | none
  compiler_wired: true | false
  migration_paths_identified: 0
  blockers_identified: 0
  last_action: [description]
  next_action: [description]
</react-modernization-state>

Output Template

The Modernization Assessment Summary — paths table, blockers table, and phased plan — is in references/assessment-output-template.md. The AI discipline rules (evidence before recommendation, quantify before scoring, incremental plans only), the anti-patterns catalog, and the error-recovery procedures live in references/discipline-and-recovery.md.

Integration with Other Skills