I have a javascript page:

var quantity_in_stock_S = 2
var quantity_in_stock_M = 2
var quantity_in_stock_L = 1
What would be the best way to take away 1 from a number using a javascript function?

I am trying to make a function so that when you click on a HTML button connected to a size it will - 1 (take 1 away) from an external javascript page.