lua equivalent of javascripts startsWith function

local startsWith = function(stringValue, searchValue, position)
    if stringValue and searchValue then
        if position then
            return string.sub(string.sub(stringValue, position + 1), 1, #searchValue) == searchValue
        else
            return string.sub(stringValue, 1, #searchValue) == searchValue
        end
    else
        return false
    end
end

-- example
-- code | expected output

startsWith("testing oof", "testing") -- true
startsWith("testing oof", "oof", 8)  -- true
                                     --
startsWith()                         -- false
startsWith("testing oof")            -- false
startsWith("testing oof", "oof")     -- false
startsWith("testing oof", "oof", 10) -- false