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kenneth
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- react-best-practices: Performance optimization patterns (client-side only) - web-design-guidelines: UI review against Web Interface Guidelines Co-authored-by: Ona <no-reply@ona.com>
38 lines
837 B
Markdown
38 lines
837 B
Markdown
---
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title: Build Index Maps for Repeated Lookups
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impact: LOW-MEDIUM
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impactDescription: 1M ops to 2K ops
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tags: javascript, map, indexing, optimization, performance
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---
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## Build Index Maps for Repeated Lookups
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Multiple `.find()` calls by the same key should use a Map.
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**Incorrect (O(n) per lookup):**
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```typescript
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function processOrders(orders: Order[], users: User[]) {
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return orders.map(order => ({
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...order,
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user: users.find(u => u.id === order.userId)
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}))
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}
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```
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**Correct (O(1) per lookup):**
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```typescript
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function processOrders(orders: Order[], users: User[]) {
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const userById = new Map(users.map(u => [u.id, u]))
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return orders.map(order => ({
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...order,
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user: userById.get(order.userId)
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}))
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}
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```
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Build map once (O(n)), then all lookups are O(1).
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For 1000 orders × 1000 users: 1M ops → 2K ops.
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