There are 2 hard things in computer science

• cache invalidation
• naming things
• off-by-one errors

Proxy Caching (proxyCache)

In Topic 4, we saw how reactive() wraps objects in a Proxy. However, a common issue in reactive programming is trying to wrap the same object multiple times, or dealing with circular references (e.g. Object A points to Object B, and Object B points back to Object A).

The Problem: Infinite Recursion

Without a cache, every time you access a nested object, the get trap would create a new Proxy wrapper.

// Without a cache:
const p1 = state.user; 
const p2 = state.user;
console.log(p1 === p2); // false! They are different "guards" for the same data.

This wastes memory and breaks object identity. Even worse, if an object points to itself, the code would keep creating Proxies until the browser crashed with a "Maximum call stack size exceeded" error.

The Solution: proxyCache

LegoDOM uses const proxyCache = new WeakMap() to keep track of every object it has already turned into a Proxy.

1. Checking the Map: At the very start of the reactive() function, the code checks: if (proxyCache.has(obj)) return proxyCache.get(obj);.

2. Storing the Result: If it's a new object, the code creates the Proxy and then immediately saves it: proxyCache.set(obj, p);.

3. The Result: If you access state.user 100 times, you get the exact same Proxy instance every time. It ensures that p1 === p2 is always true.

Why a WeakMap?

This is a critical "expert-level" choice.

• A regular Map holds a "strong reference" to its keys. If you deleted a piece of data from your state, but that data was still a key in a regular Map, the browser could never delete it from memory.

• Because proxyCache is a WeakMap, as soon as your block is destroyed and the original object is no longer needed, the browser’s Garbage Collector can automatically wipe it from the cache.

Summary of Logic:

• Step 1: Request to make obj reactive.

• Step 2: Check proxyCache. Found it? Return the existing Proxy.

• Step 3: Not found? Create a new Proxy.

• Step 4: Store obj -> Proxy in proxyCache.

• Step 5: Return the new Proxy.