L.control.scale().addTo(map);
function pixelsInOneMeterWidth() {
    var div = document.createElement('div');
    div.id="mesure";
    div.style.cssText = 'position: absolute;  left: -100%;  top: -100%;  width: 100cm';
    document.body.appendChild(div);
    var px = div.offsetWidth;
    document.body.removeChild(div);
    return px;
}

function scaleManage(){
    var pos = map.getCenter();
    var screenWidth = screen.width;
    var wFrameInPx = document.getElementById("map").clientWidth;
    var mapSpan = map.containerPointToLatLng([0, 0]).distanceTo(map.containerPointToLatLng([0, wFrameInPx]));
    var frameInMeter = wFrameInPx/pixelsInOneMeterWidth();
    return [pos,screenWidth,wFrameInPx,mapSpan, pixelsInOneMeterWidth(), frameInMeter, mapSpan/frameInMeter];
}

const widthInMeter 	  = 6378137 * 2 * Math.PI * Math.cos(map.getCenter().lat * Math.PI / 180);
var scaleOutput 	  = document.getElementById("scaleOutput");
scaleOutput.innerHTML = widthInMeter;
map.on('moveend', function(){
    scaleOutput.innerHTML = widthInMeter;
});