How to change the values ​​to the opposite mirror?

Question

JavaScript

const value = (scrollTop/trackLength).toFixed(2)
Where scrollTop is window.pageYOffset and trackLength is the total page height of the site. The values ​​of value change as you scroll FROM 0 (scroll at the top of the screen) to 1 (scroll at the very bottom).
How to change the values ​​to the opposite mirror? Those. when 0 i want 1, when 0.20 i want 0.80 and so on?

Answer 1

y = Math.abs(1 - x)