如何控制元素背景图的缩放中心？

let box = document.querySelector('.box');
let img = document.querySelector('img');
let isDown = false;
let downx = 0;
let downy = 0;
let position = {
  x: 0,
  y: 0
};
let scale = 1;

box.addEventListener('mousewheel', function (event) {
  event.stopPropagation();
  event.preventDefault();
  let rec = img.getBoundingClientRect();
  let x = (event.clientX - rec.x) / scale;
  let y = (event.clientY - rec.y) / scale;
  let delta = (event.wheelDelta ? event.wheelDelta : -event.deltaY);

  let ls = scale; // keep last scale
  scale = (delta > 0) ? (scale + 0.2) : (scale - 0.2);
  if (scale > 3 || scale < 0.8) {
    scale = ls;
    return;
  }
  let m = (delta > 0) ? 0.1 : -0.1;
  let x1 = (-x * m * 2) + (img.offsetWidth * m);
  let y1 = (-y * m * 2) + (img.offsetHeight * m);
  position.x += x1;
  position.y += y1;
  updateTransform();
});

box.addEventListener('mousedown', function (e) {
  isDown = true;
  downx = e.clientX - position.x;
  downy = e.clientY - position.y;
});

document.addEventListener('mousemove', function (e) {
  if (isDown) {
    position.x = e.clientX - downx;
    position.y = e.clientY - downy;
    updateTransform();
  }
});

box.addEventListener('mouseup', function (e) {
  isDown = false;
});

function updateTransform() {
  img.style.transform = `scale(${scale}) translate(${position.x / scale}px, ${position.y / scale}px)`;
}