Vue 2 vs Vue 3

Reactivity internals

The biggest difference between Vue 2 and Vue 3 is in the reactivity system.

Vue 2 reactivity

1. Object.defineProperty
   • Vue 2 uses Object.defineProperty to intercept property access and updates.
   • When an object is passed into data, Vue walks through each property and converts it into a reactive property.
2. Dependency tracking
   • When a template reads a reactive property, Vue records that dependency.
   • When the property changes, Vue can find the corresponding watchers and update the view.
3. Array mutation handling
   • Because Object.defineProperty cannot naturally observe every array operation, Vue 2 patches array mutation methods such as push, pop, and splice.

Vue 3 reactivity

1. ES6 Proxy
   • Vue 3 uses Proxy, which can intercept far more kinds of object operations.
2. Improved dependency tracking
   • Vue 3's tracking is more efficient and works more naturally with arrays and nested structures.
3. Composition API support
   • Vue 3 introduces the Composition API, which gives developers more flexible ways to organize logic and reactive state.

Lifecycle

Vue 3 merges some lifecycle hooks, introduces new ones, and provides more flexibility through the Composition API.

Overall, the relationship between hooks becomes clearer, and developers get better control over custom logic composition.

Performance

1. Data observation
   • Vue 2 uses Object.defineProperty() and needs to traverse properties
   • Vue 3 uses Proxy and can observe the object more directly
2. Compilation optimizations
   • Vue 3 introduces improvements such as static hoisting and cached event handlers
3. Bundle size
   • Vue 3 removes some less-used APIs and improves tree-shaking
4. Memory usage
   • Vue 3 reduces some memory-heavy behavior and improves allocation patterns
5. SSR
   • Vue 3 improves server-side rendering performance and architecture

In short, Vue 3 delivers meaningful performance improvements in initialization, rendering, and memory behavior.

Coding style

1. Options API vs Composition API
   • Vue 2 is centered around the Options API
   • Vue 3 introduces the Composition API, which makes logic more cohesive
2. defineComponent
   • Vue 3 uses defineComponent for clearer component typing and declaration
3. ref and reactive
   • Vue 3 provides direct APIs for declaring reactive variables
4. setup
   • Vue 3 centralizes component logic in setup
5. Async components
   • Vue 3 provides improved support for async component patterns

API changes

1. Options API and Composition API
   • Vue 2 mainly uses data, methods, and similar options
   • Vue 3 recommends ref, reactive, watch, computed, and related APIs
2. Filters removed
   • Vue 2 supported filters
   • Vue 3 removes them and recommends methods or computed properties instead
3. Global and instance API adjustments
   • some old APIs were removed or redesigned
4. Lower-level APIs
   • Vue 3 exposes more renderer-related extension points such as createRenderer

Diff algorithm

1. Static marking
   • Vue 2 marks static roots
   • Vue 3 uses more precise patch flags and shape flags
2. Optimization logic
   • Vue 2 uses a classic double-ended diff approach
   • Vue 3 improves patching with more refined vnode metadata and faster matching
3. Block tree optimization
   • Vue 3 can group and optimize updates more aggressively
4. Async rendering support
   • Vue 3 works better with async component rendering scenarios
5. Code reduction
   • the core update logic is more streamlined
6. SSR diff optimizations
   • Vue 3 adds improvements for server-rendered hydration and patching

Bundling and build

1. Bundle size
   • Vue 3 improves tree-shaking and removes rarely used APIs
2. Dependency optimization
   • code paths are more modular and easier for bundlers to prune
3. Build speed
   • compilation and production builds are generally improved
4. Compiler improvements
   • better caching and faster incremental feedback in many setups
5. Tree-shaking
   • more precise elimination of unused code
6. Modularity
   • Vue 3's codebase is more fully ESM-friendly
7. New built-in capabilities
   • Vue 3 supports features such as Fragment, Teleport, and Suspense

TypeScript support

1. Internal implementation
   • Vue 2 was written in JavaScript and relied on external type maintenance
   • Vue 3 was rewritten in TypeScript
2. Component definition
   • Vue 3 offers a cleaner TS experience with defineComponent
3. Options API typing
   • Vue 3 improves type inference for the Options API too
4. Composition API
   • Vue 3's Composition API works naturally with generics and TypeScript interfaces
5. Registration and props typing
   • component definitions provide better props inference and IDE help
6. TS configuration
   • Vue 3 works better with stricter TypeScript workflows
7. Built-in utility types
   • Vue 3 exports more internal and helper types for extension

Summary

Overall, Vue 3 is a full upgrade over Vue 2:

1. better performance in initialization and rendering
2. more modern features such as the Composition API, Fragment, Teleport, and Suspense
3. cleaner architecture with a TypeScript-based core
4. more efficient and flexible APIs
5. stronger support for composable and function-oriented logic organization
6. better build output and tree-shaking
7. a healthier foundation for future ecosystem growth

For teams maintaining Vue 2 projects, the migration cost still matters. But technically, Vue 3 is the more modern and capable baseline.