check the coordinates of a point in that it is inside a polygon

point-in-polygon.js

function inside(point, vs) {
    // ray-casting algorithm based on
    // https://wrf.ecse.rpi.edu/Research/Short_Notes/pnpoly.html/pnpoly.html
    // https://stackoverflow.com/questions/22521982/check-if-point-is-inside-a-polygon/29915728#29915728
    // https://github.com/substack/point-in-polygon
    
    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;
};