app.js

let count = 0
const cartIncrementButton = document.getElementById('cartIncrement')
const cartDecrementButton = document.getElementById('cartDecrement')
const countElement = document.getElementById('count')
const buyButton = document.getElementById('purchase')

function init() {
  
  countElement.innerHTML = count
  cartIncrementButton.addEventListener('click', () => {
    count++
    countElement.innerHTML = `${count}$`
  })
  
  cartDecrementButton.addEventListener('click', () => {
    if (count === 0) return 
    count--
    countElement.innerHTML = `${count}$`
  })
}

init()