Luau Type System Complete Guide — Strict Mode, Generics & Type Functions for Safe Code [2026]

Luau's type system uses Gradual Typing, letting you add type safety incrementally without breaking existing code. With the new type solver released in 2025, Luau is now a robust choice for large-scale Roblox projects.

Luau is a gradually-typed language: type annotations are optional, and unannotated variables are treated as `any`. This allows existing Roblox scripts to be migrated to typed code incrementally. Type errors are caught at development time rather than runtime, dramatically reducing debugging costs.

Set the mode with a comment on the first line of each script:

For new projects, `--!strict` is recommended. Migrate legacy code starting with `--!nonstrict`.

--!strict
local name: string = "Alice"
local score: number = 42
local isActive: boolean = true
local nickname: string? = nil  -- Nullable (string | nil)

local function greet(player: string): string
    return "Hello, " .. player
end

`string?` is syntactic sugar for `string | nil` (Optional type).

type Point = {
    x: number,
    y: number,
}

type Player = {
    name: string,
    score: number,
    position: Point,
}

Use `export type` to share aliases across modules.

local function identity<T>(value: T): T
    return value
end

local function first<T>(arr: {T}): T?
    return arr[1]
end

local n = identity(42)         -- T inferred as number
local s = identity("hello")    -- T inferred as string

Generics enable type-safe utility functions and reusable containers.

-- Union
type StringOrNumber = string | number

-- Discriminated Union
type Result<T> = { ok: true, value: T } | { ok: false, error: string }

-- Intersection
type Named = { name: string }
type Aged  = { age: number }
type Person = Named & Aged

Discriminated unions are especially useful for modeling API responses and error-handling patterns.

The new type solver introduces `read` and `write` property modifiers:

type Config = {
    read maxPlayers: number,   -- read-only
    write onUpdate: () -> (),  -- write-only
}

This allows APIs to express intent at the type level and prevents accidental mutations.

Type functions let you compute types at compile time:

type function Nullable<T>()
    return types.unionof(T, types.singleton(nil))
end

type OptionalString = Nullable<string>  -- equivalent to string | nil

This enables TypeScript-like utilities (`Partial<T>`, `Readonly<T>`) built entirely in Luau.

Enable the new solver in Studio settings; incremental migration is recommended for existing projects.

Recommendation: Luau strict mode is sufficient for small-to-medium projects. Consider roblox-ts for large teams or when TypeScript's ecosystem is essential.

Q1. Does strict mode slow down my game? No. Type checking only happens at compile/edit time and has zero runtime overhead. Q2. Do I need to convert all scripts to strict at once? No. Gradual typing lets you migrate file by file, starting with the most critical modules. Q3. Will type errors break my game? No. Type errors are warnings; the game still runs. However, fixing them is strongly recommended. Q4. Can I use generics with Roblox APIs? Yes. You can write typed wrappers such as `Instance:FindFirstChild<T>()` for safer API usage. Q5. When are type functions available? With the new type solver enabled (Roblox Studio 2025 and later). Q6. Can I mix Luau types and roblox-ts? Not within the same file. Keep them separated at the module boundary. Q7. Should I avoid `any` entirely? Yes, where possible. Prefer `unknown` with type guards for genuinely uncertain values.

Oflight supports Luau type system adoption and end-to-end Roblox game development — from architecture design to implementation and code review. Learn more at Software Development Services.