TouchList: length 屬性

const target = document.getElementById("target");

target.addEventListener("touchstart", (ev) => {
  // If this touchstart event started on element target,
  // set touch to the first item in the targetTouches list;
  // otherwise set touch to the first item in the touches list
  const touch =
    ev.targetTouches.length >= 1
      ? ev.targetTouches.item(0)
      : ev.touches.item(0);
});