Daniil
38 lines
837 B
Markdown
38 lines
837 B
Markdown
---
|
||
title: Build Index Maps for Repeated Lookups
|
||
impact: LOW-MEDIUM
|
||
impactDescription: 1M ops to 2K ops
|
||
tags: javascript, map, indexing, optimization, performance
|
||
---
|
||
|
||
## Build Index Maps for Repeated Lookups
|
||
|
||
Multiple `.find()` calls by the same key should use a Map.
|
||
|
||
**Incorrect (O(n) per lookup):**
|
||
|
||
```typescript
|
||
function processOrders(orders: Order[], users: User[]) {
|
||
return orders.map(order => ({
|
||
...order,
|
||
user: users.find(u => u.id === order.userId)
|
||
}))
|
||
}
|
||
```
|
||
|
||
**Correct (O(1) per lookup):**
|
||
|
||
```typescript
|
||
function processOrders(orders: Order[], users: User[]) {
|
||
const userById = new Map(users.map(u => [u.id, u]))
|
||
|
||
return orders.map(order => ({
|
||
...order,
|
||
user: userById.get(order.userId)
|
||
}))
|
||
}
|
||
```
|
||
|
||
Build map once (O(n)), then all lookups are O(1).
|
||
For 1000 orders × 1000 users: 1M ops → 2K ops.
|