Mastering Vue.js: The Art of Refactoring Legacy Code

Outcome-first introduction

You will finish this article with a clear, actionable roadmap to take a legacy Vue app from brittle and slow to lean and maintainable - without tearing everything down. You’ll learn how to prioritize problems, apply incremental changes safely, and measure improvements so each step clearly moves the app forward.

Why incremental refactoring beats rewrites

Rewriting is tempting. Clean slate. Modern stack. Faster developer happiness. But rewrites are expensive, risky, and often stall. Incremental refactoring gives you continuous value. Ship fixes and performance wins quickly. Reduce user-facing risk. Keep the business running. Own the codebase again.

Quick checklist (what you’ll do in this guide)

• Audit and measure: identify hotspots. Measure first, change second.
• Add safety: tests, linting, CI.
• Stabilize and modernize incrementally: compatibility builds, small API upgrades, swap state management gradually.
• Optimize for performance: code-splitting, lazy-loading, render optimizations.
• Continually measure and iterate.

1. Start with an audit: find the real problems

You cannot fix what you don’t measure. Use these tools and techniques:

• Run Lighthouse for overall page performance and UX [Lighthouse].
• Run bundle analysis (webpack-bundle-analyzer, Vite + rollup visualizer) to find large modules [webpack-bundle-analyzer], [Vite].
• Use Vue Devtools to inspect reactivity, component render counts and time spent in updates [Vue Devtools].
• Add real-user monitoring if available (Sentry, Datadog RUM).

Capture baseline metrics: bundle size, Time to Interactive, largest contentful paint, key route render times, and slow components. Without a baseline you can’t prove improvements.

2. Safety first: tests, lint, and CI

Before touching business logic, make it safe to change code:

• Add unit tests for key components and utilities using Vue Test Utils + Jest or Vitest.
• Add a handful of end-to-end tests (Cypress) for critical user journeys.
• Enforce style and correctness with ESLint + eslint-plugin-vue and Prettier.
• Run tests and lint in CI to stop regressions early.

Example: a simple unit test using Vue Test Utils + Vitest

// tests/components/Login.spec.js
import { mount } from '@vue/test-utils';
import Login from '@/components/Login.vue';

test('shows error message on submit without email', async () => {
  const wrapper = mount(Login);
  await wrapper.find('form').trigger('submit.prevent');
  expect(wrapper.text()).toContain('Email is required');
});

3. Choose a migration strategy: compatibility and incremental upgrades

If you’re on Vue 2 and aiming for Vue 3, avoid an immediate full rewrite:

• Use the Vue 2 -> Vue 3 Migration Guide to understand breaking changes [Migration Guide].
• Consider the Vue 3 compatibility build (@vue/compat) to run Vue 2 code under Vue 3 while you fix deprecations gradually [Compatibility Build].
• If staying on Vue 2 for now, still adopt modern tooling (ESLint rules, TypeScript incrementally) and plan gradual upgrades.

When migrating state management from Vuex to Pinia, do it incrementally:

• Keep Vuex running while introducing Pinia for new modules/features.
• Wrap existing Vuex-based logic with adapters or thin wrappers so components can switch to Pinia piecemeal.

Resources:

• Pinia docs and migration patterns [Pinia].
• Vue Router and other core library migration guides [Vue Router Migration].

4. Replace fragile patterns with stable alternatives

Common legacy pain points and how to fix them:

• Mixins: produce implicit props and coupling. Replace with composables (Composition API) or utility modules.

Example: mixin -> composable

// old mixin
export default {
  data() {
    return { count: 0 };
  },
  methods: {
    increment() {
      this.count++;
    },
  },
};

// composable
import { ref } from 'vue';
export function useCounter() {
  const count = ref(0);
  function increment() {
    count.value++;
  }
  return { count, increment };
}

• Large monolithic components: split by responsibility. One UI component should do one thing. Extract child components and share logic via composables.

• Heavy watchers and manual DOM manipulation: move to computed properties or composables. Let Vue manage DOM updates.

5. Performance optimizations that pay off fast

• Route-based code-splitting: lazy-load route components to shrink first bundle.

// router/index.js
const Foo = () => import(/* webpackChunkName: "foo" */ '@/views/Foo.vue');

• Component-level lazy loading and dynamic imports for rarely used widgets.
• Use keep-alive for expensive route-level components that are visited frequently.
• Use v-once on static fragments to avoid re-renders.
• Avoid expensive watchers; prefer computed. Use shallowRef/ref appropriately to prevent deep reactivity overhead.
• Virtualize large lists (vue-virtual-scroll-list or similar) to avoid rendering thousands of DOM nodes.

6. Bundler and build improvements

• Move to Vite if possible. Faster dev builds and simpler configs. Use the analysis plugins to measure bundle impact [Vite].
• Ensure tree-shaking friendly imports (import only what you need from big libraries).
• Compress and optimize assets (images, fonts). Use modern formats like WebP / AVIF.
• Add long-term caching and appropriate cache headers for static assets.

7. Gradual TypeScript adoption

TypeScript brings maintainability and better refactoring guarantees. Adopt it incrementally:

• Start with // @ts-check and JSDoc in utilities.
• Convert low-risk modules/components first.
• Use allowJs and checkJs in tsconfig to mix JS and TS during transition.

8. Observability and continuous measurement

After each change, re-run your baseline measurements. Key tools:

• Lighthouse for page performance [Lighthouse].
• Vue Devtools for render profiling [Vue Devtools].
• Bundle analyzer to verify size regressions [webpack-bundle-analyzer].

A single small optimization can look meaningless by itself. Re-measuring proves compound wins over many small changes.

9. When to consider a rewrite (and when not to)

Consider a rewrite only if:

• The app has fundamental architectural flaws that cannot be surgically fixed.
• Business can afford the time and risk and there are clear goals.

Avoid a rewrite when:

• The app can be migrated incrementally.
• You simply want modernization for developer happiness. Modernization alone doesn’t justify the risk.

10. Practical incremental roadmap (90-day plan)

Week 1–2

• Audit (Lighthouse, bundle analysis, Vue Devtools).
• Add CI jobs, linting and a small test harness.

Week 3–6

• Introduce compatibility build or guardrails.
• Start replacing the top 3 worst-performing components (split, lazy-load).

Week 7–10

• Migrate one state module to Pinia or update one large feature to Composition API.
• Add route-based splitting and verify bundle size improvements.

Week 11–12

• Measure, document findings, and plan the next quarter.

Concrete patterns and anti-patterns

Do this:

• Lazy-load features and route components.
• Encapsulate shared behaviors as composables.
• Add unit tests around critical logic before refactoring.

Avoid this:

• Changing everything at once.
• Removing tests or not measuring the impact.
• Large unreviewed merges that fix “everything.”

Useful resources

• Vue 3 Migration Guide: https://v3.vuejs.org/guide/migration/introduction.html
• Vue Compatibility Build: https://v3.vuejs.org/guide/migration/compatibility-build.html
• Pinia: https://pinia.vuejs.org/
• Vue Router migration guide: https://next.router.vuejs.org/guide/migration/index.html
• Vue Devtools: https://devtools.vuejs.org/
• Lighthouse: https://developers.google.com/web/tools/lighthouse
• webpack-bundle-analyzer: https://github.com/webpack-contrib/webpack-bundle-analyzer
• Vue codemods: https://github.com/vuejs/vue-codemods
• Vite: https://vitejs.dev/

Final takeaway

Refactoring legacy Vue apps is strategic, not surgical. Small, measured improvements compound into a big win. Ship incremental changes, protect them with tests and CI, and measure everything. Over time your codebase becomes faster, safer to change, and easier to maintain - with far less risk than a full rewrite.