b4r7x React useMemo
Overview
useMemo caches a computed value between renders. It's an optimization — code should work correctly without it. The common denominator for all valid cases: cost of computation or reference stability.
When to Use
1. Expensive computations
// ❌ Filtering 10,000 products on every render
const filtered = products.filter(p =>
p.name.toLowerCase().includes(query.toLowerCase())
);
// ✅ Recalculates only when products or query change
const filtered = useMemo(() =>
products.filter(p =>
p.name.toLowerCase().includes(query.toLowerCase())
), [products, query]
);
2. Stable reference for memo'd child
Object/array passed as props to a memo() component — without useMemo, a new reference is created every render, defeating memo.
3. Stable reference for useEffect dependency
Object/array used in useEffect deps array — without useMemo, the effect runs every render.
4. Context Provider value
const value = useMemo(() => ({ user, logout }), [user, logout]);
<AuthContext.Provider value={value}>
This is one of the few cases where useMemo is justified without profiling first — the mechanism is well-understood and predictable.
When NOT to Use
| Scenario | Why skip |
|---|---|
Simple operation (price * quantity) |
useMemo overhead > computation cost |
| Primitives (string, number, boolean) | React compares by value, not reference |
| Dependencies change every render | Memoization never caches — just overhead |
| No measured performance problem | Premature optimization obscures code |
| First render | useMemo only helps on re-renders, never first render |
Decision Heuristic
Slow render? → Measure with React DevTools Profiler
↓
Is THIS component the bottleneck?
↓ yes ↓ no
Is computation expensive Look higher up
or result is object/array
passed to memo'd child?
↓ yes
Add useMemo
React Compiler (2025+)
React Compiler auto-memoizes computed values at build time. If you're using it, don't add manual useMemo unless you hit a measured issue or work with uncompiled external libraries. Same rules as useCallback — see react-usecallback for details.
Key Rule
React docs say: check if you have a logic bug first — useMemo on buggy code just hides it. Measure first, optimize second.
References
- useMemo — React docs — official API reference with when-to-use guidance
- Understanding useMemo and useCallback — Josh Comeau's practical guide to memoization decisions
- How to use memo and useCallback — Nadia Makarevich on reference stability