How do i use this output value as a input

I want to do math.random and put that value in this script

let bday = Math.floor(
  Math.random() * (30 - 1 + 1) + 1
);

await test.page.select('select[name="day_select"]',bday);

but the problem is I need to add '' to bday, cuz if I do

await test.page.select('select[name="day_select"]','10');

it works, but this

await test.page.select('select[name="day_select"]',bday);

and this

await test.page.select('select[name="day_select"]','bday');

doesn't work