두 요소 스크롤 동기화

const box1 = document.getElementById('box1')
const box2 = document.getElementById('box2')

let isSyncing = false

function syncScroll(source, target, sourceWidth, targetWidth) {
  const ratio = source.scrollLeft / (source.scrollWidth - sourceWidth)
  target.scrollLeft = ratio * (target.scrollWidth - targetWidth)
}

box1.addEventListener('scroll', () => {
  if (!isSyncing) {
    isSyncing = true
    syncScroll(box1, box2, 1280, 640)
    isSyncing = false
  }
})

box2.addEventListener('scroll', () => {
  if (!isSyncing) {
    isSyncing = true
    syncScroll(box2, box1, 640, 1280)
    isSyncing = false
  }
})

isSyncing 플래그로 무한 이벤트 루프 방지