npm install leaflet-lasso

<script src="https://unpkg.com/[email protected]/dist/leaflet-lasso.umd.min.js"></script>

import "leaflet-lasso";

interface LassoHandlerOptions {
    polygon?: L.PolylineOptions,
    intersect?: boolean;
}

const lasso = L.lasso(map, options);
yourCustomButton.addEventListener('click', () => {
    lasso.enable();
});

type LassoControlOptions = LassoHandlerOptions & L.ControlOptions;

L.control.lasso(options).addTo(map);

interface LassoHandlerFinishedEventData {
    latLngs: L.LatLng[];
    layers: L.Layer[];
}

map.on('lasso.finished', (event: LassoHandlerFinishedEventData) => {
    console.log(event.layers);
});