1---
2title: Build Index Maps for Repeated Lookups
3impact: LOW-MEDIUM
4impactDescription: 1M ops to 2K ops
5tags: javascript, map, indexing, optimization, performance
6---
7 
8## Build Index Maps for Repeated Lookups
9 
10Multiple `.find()` calls by the same key should use a Map.
11 
12**Incorrect (O(n) per lookup):**
13 
14```typescript
15function processOrders(orders: Order[], users: User[]) {
16  return orders.map(order => ({
17    ...order,
18    user: users.find(u => u.id === order.userId)
19  }))
20}
21```
22 
23**Correct (O(1) per lookup):**
24 
25```typescript
26function processOrders(orders: Order[], users: User[]) {
27  const userById = new Map(users.map(u => [u.id, u]))
28 
29  return orders.map(order => ({
30    ...order,
31    user: userById.get(order.userId)
32  }))
33}
34```
35 
36Build map once (O(n)), then all lookups are O(1).
37For 1000 orders × 1000 users: 1M ops → 2K ops.
38