Leaflet function to get a layer at a certain point. Supports polygons and multipolygons with holes.

getlayeratpoint.js

function pointInRing(point, vs)
{
    var x = point[0], y = point[1];

    var inside = false;
    for (var i = 0, j = vs.length - 1; i < vs.length; j = i++) {
        var xi = vs[i][0], yi = vs[i][1];
        var xj = vs[j][0], yj = vs[j][1];
        
        var intersect = ((yi > y) != (yj > y))
            && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);
        if (intersect) inside = !inside;
    }
    
    return inside;
}

function pointInPolygon(point, polygon)
{
    // Test if it is inside the polygon
    if (!pointInRing(point, polygon[0]))
        return false;

    // Test if it is inside a hole
    for (var i = 1; i < polygon.length; ++i)
        if (pointInRing(point, polygon[i]))
            return false;

    return true;
}

function pointInMultiPolygon(point, multipolygon)
{
    for (var i = 0; i < multipolygon.length; ++i)
        if (pointInPolygon(point, multipolygon[i]))
            return true;

    return false;
}

function getLayerAtPoint(latlng, layergroup)
{
    var p = [latlng.lng, latlng.lat];
    
    for (var layer_id in layergroup._layers) {
        var layer = layergroup._layers[layer_id];
        var type = layer.feature.geometry.type;

        if (['Polygon', 'MultiPolygon'].indexOf(type) === -1)
            continue;

        var coordinates = layer.toGeoJSON().geometry.coordinates;

        // normalize to MultiPolygon
        if (type == 'Polygon')
            coordinates = [coordinates];

        if (pointInMultiPolygon(p, coordinates))
            return layer;
    }

    return null;
}

var layergroup = L.geoJSON(...);
var layer = getLayerAtPoint(marker.getLatLng(), layergroup);