Determine if a point reside inside a leaflet polygon

Use the Ray Casting algorithm for checking if a point (marker) lies inside of a polygon:

function isMarkerInsidePolygon(marker, poly) {
    var polyPoints = poly.getLatLngs();       
    var x = marker.getLatLng().lat, y = marker.getLatLng().lng;

    var inside = false;
    for (var i = 0, j = polyPoints.length - 1; i < polyPoints.length; j = i++) {
        var xi = polyPoints[i].lat, yi = polyPoints[i].lng;
        var xj = polyPoints[j].lat, yj = polyPoints[j].lng;

        var intersect = ((yi > y) != (yj > y))
            && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);
        if (intersect) inside = !inside;
    }

    return inside;
};

See jsfiddle for example.

Original source for the code: https://github.com/substack/point-in-polygon/blob/master/index.js

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See also 2014's similar answer, https://stackoverflow.com/a/41138512/287948

Here is the modified(with @Sumit hints) version of @gusper answer that worked for me:(i had donuts)

function isMarkerInsidePolygon(marker, poly) {
    var inside = false;
    var x = marker.getLatLng().lat, y = marker.getLatLng().lng;
    for (var ii=0;ii<poly.getLatLngs().length;ii++){
        var polyPoints = poly.getLatLngs()[ii];
        for (var i = 0, j = polyPoints.length - 1; i < polyPoints.length; j = i++) {
            var xi = polyPoints[i].lat, yi = polyPoints[i].lng;
            var xj = polyPoints[j].lat, yj = polyPoints[j].lng;

            var intersect = ((yi > y) != (yj > y))
                && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);
            if (intersect) inside = !inside;
        }
    }

    return inside;
};

I found that none of the above answers worked for counting markers within non-contiguous polygons. Here's an example polygon where the above functions returned 0 markers inside:

For anyone who needs to do that, the Leaflet.PointInPolygon package worked for me: https://github.com/hayeswise/Leaflet.PointInPolygon

It's a bit slow, but it seems to be accurate.