| name | roblox-luau-types |
| description | Luau type system: annotations, generics, narrowing, inference philosophy, sealed/unsealed tables, exports, and Roblox-aware typing.
|
| last_reviewed | "2026-05-27T00:00:00.000Z" |
Luau Type System
When to Load
Load this skill when working with Luau type annotations, generics, union types, type narrowing, or sealed/unsealed table behavior. Also load when choosing strictness modes (--!strict vs --!nonstrict), designing module type exports, or typing metatable-backed objects.
Quick Reference
Strictness: --!strict (new code), --!nonstrict (transitional), --!nocheck (legacy only). The New Type Solver (GA Nov 2025) is faster/more accurate.
Inference philosophy: Infer first, annotate boundaries (params, returns, exports). Don't annotate every local — noise hides signal.
Sealed vs unsealed tables:
local t = {} -- unsealed: can add fields
t.x = 1 -- OK
local t: {x: number} = {x=1} -- sealed: no new fields
t.y = 2 -- ERROR
Build tables fully before annotating. Passing/returning seals them.
Unions & tagged unions:
local id: string | number = "abc"
type State<T> = {kind:"loading"} | {kind:"ready", value:T} | {kind:"fail", msg:string}
-- Discriminate: if state.kind == "ready" then state.value is narrowed
Narrowing:
if typeof(x) == "string" then ... end -- primitive narrowing
if inst:IsA("BasePart") then ... end -- Instance narrowing
assert(val, "missing") -- non-nil narrowing
Generics: Use when input→output type matters. function first<T>(list: {T}): T?. Generic aliases: type Result<T> = {success: boolean, value: T?}. Never replace with any.
Type exports: export type Foo = {...} at module boundary. Consumers use require + Types.Foo.
Object typing: export type Counter = typeof(setmetatable({} :: CounterData, Counter)) for precise self.
Casts (::): Precision tool to narrow overly generic inference — never to hide errors.
Key mistakes: Unsealed any propagation in nonstrict, sealing tables too early, unions without discriminants, annotating every local.
Full reference: see references/full.md
