felt/static/map.js

let map = null;
let mapImg = null;
let tokens = [];

function initializeMap(mapImgUrl) {
  if (!map) {
    map = L.map('map', { minZoom: 0, maxZoom: 4, crs: L.CRS.Simple });
    map.on("zoomend", resizeMarkers);
  }
  if (mapImg) {
    mapImg.removeFrom(map);
  }
  mapImg = L.imageOverlay(mapImgUrl, [[-180, 180],[180, -180]]);
  mapImg.addTo(map);
  map.setMaxBounds([[-180,180],[180,-180]]);
  map.setView([0,0], 2);
  while (tokens.some(t=>t)) {
    tokens[0].m.removeFrom(map);
    tokens.shift();
  }
}

// this works but assumes the map is square (reasonable limitation I think)
function resizeMarkers() {
  tokens.forEach(t=>{
    const icon = t.m.options.icon;
    const scaleFactor = mapImg._image.clientWidth / mapImg._image.naturalWidth;
    icon.options.iconSize = [scaleFactor * t.sz[0], scaleFactor * t.sz[1]];
    t.m.setIcon(icon);
  });
}

function addToken(token) {
  const self = { sz: token.sz, m: L.marker(token.pos, {
    icon: L.icon({
      iconUrl: token.img,
      iconSize: token.sz,
    }),
    title: token.name,
    draggable: true,
    autoPan: true
  })};
  
  tokens.push(self);
  self.m.addTo(map);
}

// token for testing in browser console
const t = {
  img: "https://nilfm.cc/favicon.png",
  sz: [32,32],
  pos: [0,0],
  name: "test",
}