Build Index Maps for Repeated Lookups

Build Index Maps for Repeated Lookups

Impact: MEDIUM (1M ops to 2K ops for joined data)

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

Incorrect (O(n) per lookup):

function renderFeed(posts: Post[], users: User[]) {
  return posts.map(post => ({
    ...post,
    author: users.find(u => u.id === post.authorId)
  }))
}

Correct (O(1) per lookup):

function renderFeed(posts: Post[], users: User[]) {
  const userById = new Map(users.map(u => [u.id, u]))

  return posts.map(post => ({
    ...post,
    author: userById.get(post.authorId)
  }))
}

Build map once (O(n)), then all lookups are O(1). For 1000 posts x 1000 users: 1M ops -> 2K ops.

Common React Native cases:

• Joining feed posts with user data for FlatList rendering
• Resolving category names for task items
• Mapping notification IDs to read/unread status

Source: vercel-labs/agent-skills (MIT License)