Slideshow previous and next mod operator

mod-slideshow.js

/**
 * For some reason I'm always forgetting this.
 * Probably because I'm not a mathematician. 
 */
let counter = 0

const prev () => {
  counter -= 1
}

const next () => {
  counter += 1
}

const slide = () => {
  let index = mod(counter, slides.length)
  return slide = slides[index]
}

const mod = (num, mod) => {
  let remain = num % mod
  return Math.floor(remain >= 0 ? remain : remain + mod)
}