Functions, Scope & Object Model

Map, Set, WeakMap & WeakSet

Map and Set are purpose-built collections for key-value lookup and uniqueness. WeakMap and WeakSet hold object keys weakly, making them useful for metadata that should not prevent garbage collection.

Map is better than object for arbitrary key-value collections
Set models uniqueness and membership
WeakMap keys are weakly held objects
WeakMap is excellent for private metadata
Weak collections are not iterable

Collection choice
Collection	Keys / values	Iterable?	Best use
Map	Any key → value	Yes	Dynamic dictionaries and lookup tables
Set	Unique values	Yes	Membership and deduplication
WeakMap	Object key → value	No	Object metadata without preventing GC
WeakSet	Object values	No	Track object membership weakly

Map and Set basics

const usersById = new Map(users.map((user) => [user.id, user]))
const user = usersById.get("u_123")

const seen = new Set()
for (const item of items) {
  if (seen.has(item.id)) continue
  seen.add(item.id)
  process(item)
}

Use Map for lookup and Set for membership instead of repeatedly scanning arrays.

WeakMap metadata

const metadata = new WeakMap()

function attachMetadata(node, info) {
  metadata.set(node, info)
}

function getMetadata(node) {
  return metadata.get(node)
}

// When `node` becomes unreachable elsewhere, WeakMap does not keep it alive.

WeakMap is useful when the object owns the lifetime, not the metadata table.