famapp/.agents/skills/react-best-practices/upstream/rules/js-index-maps.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Build Index Maps for Repeated Lookups	LOW-MEDIUM	1M ops to 2K ops	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):

function processOrders(orders: Order[], users: User[]) {
  return orders.map((order) => ({
    ...order,
    user: users.find((u) => u.id === order.userId),
  }));
}

Correct (O(1) per lookup):

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.