Functions, Scope & Object Model

Arrays, Collections & Immutability

Arrays are ordered indexed objects; Maps and Sets provide clearer collection semantics for key-value lookup and uniqueness. Immutable update patterns reduce accidental shared-state bugs.

Arrays preserve order and support indexed access
Objects are records, Maps are collections
Sets model uniqueness
Immutable updates preserve old references
Shallow copies are shallow

Collection choice
Need	Use	Reason
Ordered sequence	Array	Indexing and order
Known object shape	Object	Named properties
Dynamic key-value collection	Map	Arbitrary keys and clear size/iteration
Unique values	Set	Membership and dedupe
Object metadata without preventing GC	WeakMap	Weak object keys

Immutable update by id

const nextUsers = users.map((user) =>
  user.id === changed.id ? { ...user, ...changed } : user
)

const usersById = new Map(nextUsers.map((user) => [user.id, user]))

Create a new array and new changed object; reuse unchanged objects.