Senior LevelJavaScript & TypeScript
How to implement custom immutability utilities or structural sharing (similar to Immer) in JS/TS?
❓ Interview Prompt: Explain Copy-On-Write (COW) pattern, Proxy-based draft mutation tracking, and structural sharing.
Immutability libraries like Immer allow writing code that mutates a temporary draft object while producing an immutable updated state tree via Structural Sharing.
Structural Sharing & Copy-On-Write (COW):
Instead of deep cloning an entire object tree on every small change:
1
Wrap the state object in a Proxy draft.
2
Track modified property branches.
3
Copy ONLY the modified nodes (Copy-On-Write), while reusing unchanged object references directly in the new state tree!
Code Implementation Example
javascript
// Simplified Proxy Copy-On-Write (COW) draft helper
function produce(baseState, recipe) {
const copies = new Map();
const handler = {
get(target, prop) {
return target[prop];
},
set(target, prop, value) {
if (!copies.has(target)) {
copies.set(target, { ...target }); // Copy on write!
}
const copy = copies.get(target);
copy[prop] = value;
return true;
}
};
const draft = new Proxy(baseState, handler);
recipe(draft);
return copies.get(baseState) || baseState;
}💡Proxy-based Copy-On-Write state draft producer with structural sharing.
🎯 Interview Key Takeaway
Structural sharing reuses unchanged object references during immutable updates, maintaining high performance and memory efficiency.
#Immutability#Immer#Structural Sharing#Senior
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