How does Vue 3's Reactivity System work under the hood?

Vue 3 uses ES6 Proxies to intercept object operations and track dependencies via a Global WeakMap.

When a reactive object is accessed (getter), Vue's 'track' function records the current 'effect' (the function running, like a render or watcher). When the object is modified (setter), the 'trigger' function finds all stored effects for that property and executes them. Because it uses Proxies, it can detect property additions/deletions and array index changes, which was a major limitation in Vue 2's Object.defineProperty.