Explicit Type Annotations

Part of the Introduction To Luau section of Coddy's Lua journey — lesson 11 of 73.

Knowing when to write annotations and when to lean on inference is a core Luau skill. The most common case that needs an explicit annotation: declaring a variable without an initial value.

local score: number   -- no value yet — annotate the intent
score = 85            -- assigned later, checked against number

With no initializer there's no value to infer from. In the default nonstrict mode an unannotated, uninitialized variable is treated as any — losing the safety you came to Luau for. The explicit : number locks the intended type from the start, so only numbers can ever be assigned:

local score: number
score = "85" -- ✗ type error: string is not a number

At runtime, remember, a declared-but-unassigned variable simply holds nil until the assignment happens — the annotation documents and enforces what it will hold.

Annotations also shine wherever the type isn't obvious from the code: function parameters, return types, and empty tables you'll fill later. Rule of thumb — infer when the value is right there, annotate when it isn't.

Challenge

Easy

Declare a variable named totalScore with the explicit type number, without assigning an initial value. On the next line, assign it the value 95.

Then declare a variable named playerName with the explicit type string, again without an initial value. On the next line, assign it the value "Alex".

Finally, print both variables on separate lines — first totalScore, then playerName.

Try it yourself

-- Write code here
-- 1) declare totalScore: number (no value), then assign 95
-- 2) declare playerName: string (no value), then assign "Alex"
-- 3) print totalScore, then playerName

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Explicit Type Annotations

All lessons in Introduction To Luau