var SelectArea = require('leaflet-area-select');
// or
import SelectArea from 'leaflet-area-select';

let map = new L.Map('map', {
  selectArea: true // will enable it by default
});

// or
map.selectArea.enable();

map.on('areaselected', (e) => {
  console.log(e.bounds.toBBoxString()); // lon, lat, lon, lat
});

// You can restrict selection area like this:
const bounds = map.getBounds().pad(-0.25); // save current map bounds as restriction area
// check restricted area on start and move
map.selectArea.setValidate((layerPoint) => {
  return bounds.contains(
    this._map.layerPointToLatLng(layerPoint)
  );
});

// now switch it off
map.selectArea.setValidate();

// dragging will be enabled and you can 
// start selecting with Ctrl key pressed
map.selectArea.setCtrlKey(true); 

// box-zoom will be disabled and you can 
// start selecting with Shift key pressed
map.selectArea.setCtrlKey(true);