Variadic Functions

You've used Lua's ... (varargs) to write functions that accept any number of arguments. Luau lets you type the varargs, so every argument that flows in is checked:

function sum(...: number): number
    local total = 0
    for _, n in ipairs({...}) do
        total = total + n
    end
    return total
end

print(sum(1, 2))           -- 3
print(sum(1, 2, 3, 4, 5))  -- 15

...: number declares that every extra argument must be a number: sum(1, "two") becomes a type error. The runtime tools are the ones you know:

• {...} collects the varargs into an array (here a {number})
• select("#", ...) counts how many arguments arrived
• ... can be passed straight through to another function, e.g. math.max(...)

As in Lua, ... must be the last parameter: regular (typed) parameters come first, the variadic tail collects the rest:

function joinWords(separator: string, ...: string): string
    return table.concat({...}, separator)
end

print(joinWords("-", "a", "b", "c"))  -- a-b-c