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Functions are first-class. Two equivalent syntaxes:

function add(a, b)
    return a + b
end

-- same as:
add = function(a, b)
    return a + b
end

Prefer local function for module-private helpers — same as local x = function ....

Multiple return values are first-class. Lua functions can return any number of values, and assignments take what they need:

function divmod(a, b)
    return a // b, a % b
end

local q, r = divmod(17, 5)   -- q=3, r=2

Variable arguments with ...:

function sum(...)
    local total = 0
    for _, v in ipairs({...}) do
        total = total + v
    end
    return total
end
print(sum(1, 2, 3, 4))  -- 10