js-index-maps

Multiple .find() calls by the same key should use a Map.

Skills
Web development
4

What it does and how to use it

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Install (Claude Code)

mkdir -p .claude/skills/js-index-maps && curl -fsSL https://doutorai.pt/data/lib/skills__js-index-maps.md -o .claude/skills/js-index-maps/SKILL.md

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Content

---
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 userBy

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