Optional Parameters

In plain Lua you could always omit arguments: the missing ones silently became nil, and forgetting to handle that was a classic source of bugs. Luau makes the possibility explicit: add ? to a parameter's type to mark it optional.

function greet(name: string, title: string?): string
    if title then
        return `{title} {name}`
    end
    return `Hello, {name}`
end

print(greet("Ada"))          -- Hello, Ada
print(greet("Ada", "Dr."))   -- Dr. Ada

string? reads as "string or nil". Callers may pass a string or skip the argument entirely; parameters without ? are required, and the checker flags calls that omit them.

Inside the function, an optional parameter might be nil, so check it before using it: if title then ... end is the standard pattern. Skip the check and concatenate nil into a string, and you get the same runtime error Lua always gave you; the type checker warns you first.