Js Index Maps

---
name: js-index-maps
description: Build Index Maps for Repeated Lookups. Use when you need help with js index maps.
license: CC-BY-NC-SA-4.0
metadata:
  risk: unknown
  source: community
  kind: mode
  category: rules
---

## 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.