vue-modernization-analyzer

name	vue-modernization-analyzer
audience	team
description	Analyzes legacy Vue codebases and produces actionable modernization plans. Primary migration paths include Options API to Composition API, Vue 2 to Vue 3, Vue CLI to Vite, JavaScript to TypeScript, Vue Test Utils/Karma/Mocha to Vitest + Vue Testing Library, legacy Vuex to Pinia, and removed-in-Vue-3 pattern cleanup (filters, event bus, `$listeners`). Does NOT perform the migration — assesses, quantifies risk, and plans.

Vue 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 Vue codebases accumulate technical debt in predictable patterns: Options API components with sprawling methods/computed/watch blocks that the Composition API expresses more cohesively, Vue CLI's unmaintained and slow build, Vue 2 behind on Vue 3's performance and Composition API, untyped JavaScript where TypeScript would catch whole bug classes, Karma/Mocha + Vue Test Utils suites coupled to implementation, and a Vuex store that predates Pinia's simpler, typed API.

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 vue-feature-slice / vue-app-scaffolder).

Grounding note: the KB has a Vue 2/3 corpus under collection="javascript" (alongside the JS→TS path). Cite vuejs.org (migration guide) + Vite/Vitest docs for version-specific detail. Never invent a vue 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 Vue 3 upgrade blocked by an unmaintained UI library is not viable
5. Every recommendation must cite evidence — "this looks like Options API sprawl" is not evidence; grep -rln "export default {" src/ | xargs grep -l "methods:" is

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

# Vue + tooling versions
grep -E '"(vue|@vue/cli-service|typescript|vite|vue-test-utils|vuex|pinia)"' package.json

# Options API surface (Options→Composition surface)
grep -rln "export default {" src/ | xargs grep -l "methods:\|data()" | wc -l

# JavaScript vs TypeScript surface
find src -name "*.vue" | xargs grep -L 'lang="ts"' | wc -l   # JS-script SFCs to migrate
find src -name "*.ts" | wc -l                                 # already TS

# Build tooling
grep -l "@vue/cli-service" package.json && echo "Vue CLI — migrate to Vite"

# Deprecated/removed-in-3 patterns
grep -rn "filters:\|Vue.filter\|\$listeners\|\$on(\|\$off(\|\$once(" src/ | head

# Test framework
grep -rln "from '@vue/test-utils'" src/ | wc -l
grep -rln "@testing-library/vue" src/ | wc -l

# Ad-hoc data fetching
grep -rn "created()\|mounted()" src/ | wc -l
grep -rn "onMounted" src/ | grep -c "fetch\|axios"

Phase 2: ASSESS

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

Path	Tool / Signal	Assessment Method
Options → Composition API	methods:/data() block count	Count Options API components; weight by watcher/computed complexity
Vue CLI → Vite	@vue/cli-service present	Check vue.config.js customizations; map env vars (VUE_APP_ → VITE_)
Vue 2 → Vue 3	vue major version	Map breaking changes (filters, $listeners, global API); check every dep's Vue 3 support; find Vue Test Utils v1
JS → TS	.vue files without lang="ts" count	Gradual adoption viability; per-file conversion
Vue Test Utils v1/Karma → Vitest + Vue Testing Library	@vue/test-utils v1 import count	Test-by-test; Vue 2 test tooling is a hard blocker for Vue 3
Vuex → Pinia	store module count	Per-module migration; separate server-state extraction
Ad-hoc fetch → query layer	created()/mounted()+fetch count	Identify fragile fetching with no caching/dedupe

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

Phase 3: PLAN

Objective: Produce a prioritized, phased modernization plan.

1. Phase 0: Prerequisites — component tests characterizing current behavior, CI, Node/version pinning (if missing)
2. Phase 1: Tooling wins — Vue CLI → Vite; introduce TypeScript per-file
3. Phase 2: Test framework — Vue Test Utils v1/Karma → Vitest + Vue Testing Library (unblocks the version bump)
4. Phase 3: Version — Vue 2 → Vue 3, fixing breaking changes (filters, $listeners, global API)
5. Phase 4: Components — Options API → Composition API, slice by slice
6. Phase 5: State & data — Vuex → Pinia; ad-hoc fetch → query layer
7. Phase 6: Types — finish JS → TS, turn on strict

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

<vue-modernization-state>
  phase: SCAN | ASSESS | PLAN | REPORT | COMPLETE
  vue_version: [2 / 3 / unknown]
  bundler: vue-cli | vite | webpack-custom | nuxt | unknown
  language: js | mixed | ts
  ts_strict: true | false | n/a
  options_api_components: [count]
  test_framework: vtu-v1 | vitest-rtl | mixed | none
  state_lib: vuex | pinia | none
  migration_paths_identified: 0
  blockers_identified: 0
  last_action: [description]
  next_action: [description]
</vue-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

Skill	Relationship
vue-app-scaffolder	When the plan includes Vue CLI → Vite, scaffold the target Vite + TS skeleton and migrate into it.
vue-feature-slice	Modernized components are reorganized into feature slices; use during the component phase.
vue-architecture-checklist	Run after modernization to verify the new structure meets quality gates.
vue-security-review	Run after the upgrade — new Vue/deps have different security characteristics.
react-modernization-analyzer / python-modernization-analyzer / rust-migration-analyzer	Sibling analyzers; identical assess-don't-execute philosophy, different stacks.