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String Enums as Unions

Part of the Introduction To Luau section of Coddy's Lua journey. Lesson 57 of 80.

Numbers make cryptic logs: seeing 2 tells you little, seeing "down" tells you everything. For readable enums, Luau's literal union types are the tool: a type whose members are exact strings:

type Direction = "up" | "down" | "left" | "right"

local facing: Direction = "up"    -- ✓
local typo: Direction = "upp"     -- ✗ type error while editing

A variable annotated with Direction can hold those four strings and nothing else. Misspell one and the checker catches it instantly: the bug never reaches a player.

At runtime the values are plain strings. Print them, concatenate them, compare them with ==. Everything you know about strings applies:

print(facing)            -- up
print(`facing {facing}`) -- facing up

Compare the two patterns you now have: the constant table creates one runtime value (UserRole) holding members, while the literal union creates no value at all. There is no Direction table, only a rule the checker enforces about which strings fit. Descriptive, self-documenting values with zero runtime cost make literal unions the go-to enum style in Luau code.

challenge icon

Challenge

Easy

Store a direction that can only be one of four names.

  1. Write type Direction = "up" | "down" | "left" | "right".
  2. Declare local facing: Direction = "left".
  3. Print facing, then `The player moves {facing}`.

Press Run Code: the output shows left and The player moves left.

Try it yourself

-- 1) type Direction as a union of four strings
-- 2) local facing: Direction = "left"
-- 3) print facing, then the sentence

All lessons in Introduction To Luau

1Getting Started with Luau

What Is Luau?Why Use Luau?Your First Luau CodeType Checking & Error ModesRecap: Introduction to Luau

4Working with Functions

Typing Params & Return ValuesTyping Anonymous FunctionsFunctions Returning NothingOptional ParametersDefault Parameter ValuesDefaults for BooleansVariadic FunctionsDefining Function TypesRecap: Typed Functions

7Project: Typed Task Manager

Project: The Task ShapeFunction to Add a TaskProject lessonFunction to Change Task StatusProject lessonFunction: List Tasks by StatusProject lessonFunction: Print Task SummaryProject lessonCommand Loop: AddProject lessonCommand Loop: List and DoneProject lessonCommand Loop: SummaryProject lesson

10Project: Generic Inventory

Project: Inventory ItemFunction: Add ItemsProject lessonThe Weapons ListProject lessonFunction: Find Item By IDProject lessonSpecific Inventory TypesProject lessonFunction: Describe ItemsProject lessonAnswer Item QueriesProject lesson

2Core Types

Number, String and BooleanThe 'any' Type: Escape HatchThe 'unknown' TypeNil & Optional TypesType Inference in ActionExplicit Type AnnotationsRecap: Core Types Practice

5Aliases, Unions, Intersections

Type Aliases for PrimitivesUnion TypesWorking with Union TypesLiteral TypesIntersection TypesCombining Type AliasesRecap: Advanced Type Combos

8Enums, the Luau Way

The Enum Pattern in LuauNumeric Enums with TablesString Enums as UnionsUsing Literal Union EnumsFreezing Constant TablesRecap: Enums, the Luau Way

11Advanced Topics

Type AssertionsType Guards With typeofThe never TypeNil Safety In Strict ModeIndex SignaturesRecap: Config Value TypesRecap: Fine-Tuning Types

3Typed Tables: Arrays & Maps

Typed ArraysAdding and Reading ElementsWhat is a Map Type?Declaring and Accessing MapsIterating TablesMixed-Shape TablesReading a GridBuilding a Gridtable.unpackRecap: Arrays and Maps

6Typing Table Shapes

Inline Shape AnnotationsType Aliases for ShapesOptional PropertiesShapes vs. Loose TablesExtending ShapesAdding Methods to ShapesSelf and Colon MethodsRecap: Defining Table Shapes

9Generics: A First Look

The Problem Generics SolveA Generic Identity FunctionUsing a Generic FunctionGeneric ArraysGeneric Type AliasesRecap: Generic Functions

Practice on your own: Online Luau compiler