Subchapter 3.34

Js Index Maps

rules/js-index-maps.mdMarkdown837 BView on GitHub

Build Index Maps for Repeated Lookups

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

Incorrect (O(n) per lookup):

Js Index Maps · Vercel React Best Practices · vercel-labs/agent-skills · Skills Docs

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

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)
  }))
}