functional

Use spread syntax and non-mutating methods instead of push, splice, or sort. For example, add items with [...items, newItem], remove by index with slice combinations, and sort with [...items].sort() to copy first.

Return new objects with the spread operator instead of mutating arguments, and mark data structures with readonly properties and ReadonlyArray<T> so the TypeScript compiler rejects mutation attempts at compile time.

Use readonly on object properties whose contract is immutable, and ReadonlyArray<T> for arrays callers must not mutate. Local encapsulated implementation state can stay mutable when it does not leak into the domain contract.

This Skill recommends Functional Light principles: pure functions, immutable data, and composition without heavy libraries. Avoid monads, fp-ts, and Ramda unless the project explicitly requires them, since they add abstraction cost.

Use an options object when parameters form a meaningful group, several values share the same type, or optional arguments make ordering unclear. Keep simple positional APIs like add(a, b) positional when the order is obvious.

Use a Result type when expected failures are part of the caller-facing contract and callers must handle both branches explicitly. Preserve existing exception or nullable-value conventions when they communicate the contract more clearly.