Luau Type System
When to Load
Load for Luau types: annotations, generics, unions, narrowing, sealed/unsealed tables, strictness (--!strict vs --!nonstrict), module type exports, metatable typing, and new-solver features (keyof, setmetatable<T, M>, type functions, read). For syntax, roblox-luau-core; for OOP/async, roblox-luau-patterns.
Quick Reference
Strictness: --!strict for maintained code, --!nonstrict while transitioning, --!nocheck only for legacy or generated code. Directives and project settings select the mode; never assume one global default.
New solver gate: keyof, rawkeyof, setmetatable<T, M>, type functions, and read members need the new type solver. On by default for nocheck/nonstrict projects; strict projects need Workspace.UseNewLuauTypeSolver = Enabled (Scripting). Errors here usually mean wrong solver, not wrong syntax.
Inference philosophy: Infer first; annotate boundaries (params, returns, exports). Don't annotate every local.
Sealed vs unsealed tables: An unannotated local table stays open to new fields until annotated, returned from a function, or its scope exits; annotation seals it. Build tables fully before annotating; passing a table to a typed function does not seal it.
Unions: local id: string | number is a union; prefer tagged unions (type State<T> = {kind:"loading"} | {kind:"ready", value:T}) and discriminate on kind to narrow.
Narrowing: typeof(v) == "string" narrows primitives, instance:IsA("BasePart") narrows Instances, assert(v, "msg") removes nil. Discriminants narrow tagged unions; full reference has worked examples.
Generics: Use when input→output type matters: function first<T>(list: {T}): T?, or type Result<T> = {success: boolean, value: T?}. Never replace with any.
Type exports: export type Foo = {...} at module boundaries; consumers use Types.Foo.
Object typing: typeof(setmetatable({} :: CounterData, Counter)) types precise self; the new solver adds setmetatable<T, M> directly.
Casts (::): For narrowing overly generic inference, never for hiding errors.
Trust boundaries: Annotations contract with the compiler, not runtime data. Remotes, DataStores, HttpService, and attributes still need checks; inside a trusted boundary, let types carry the load.
Key mistakes: any propagation in nonstrict, sealing too early, unions without discriminants, annotating every local, deleting new-solver syntax without checking the solver setting.
Full reference: see
references/full.md
