a = "10"

local function ToInteger(number)
    return math.floor(tonumber(number) or error("Could not cast '" .. tostring(number) .. "' to number.'"))
end

local a = "10"
print(type(a))
local num = tonumber(a)
print(type(num))

string
   number

a=tonumber(S)

a + 0 -- forces the conversion into float, due to how + works

a | 0 -- (| is the bitwise or) forces the conversion into integer.
--However, unlike math.tonteger, it errors if it fails

x = tonumber("10")

local stringnumber = "10"
local a = tonumber(stringnumber)
print(a + 10)

output:

20

local stringnumber = "10"

local intnumber = stringnumber * 1