JavaScript implementation of winding number algorithm to determine whether a point is inside a polygon

point_in_polygon_using_winding_number.js

//JavaScript implementation of winding number algorithm to determine whether a point is inside a polygon
//Based on C++ implementation of wn_PnPoly() published on http://geomalgorithms.com/a03-_inclusion.html

function pointInPolygon(point, vs) {
  const x = point[0], y = point[1];
  let wn = 0;

  for (let i = 0, j = vs.length - 1; i < vs.length; j = i++) {
    let xi = vs[i][0], yi = vs[i][1];
    let xj = vs[j][0], yj = vs[j][1];

    if (yj <= y) {
      if (yi > y) {
        if (isLeft([xj, yj], [xi, yi], [x,y]) > 0) {
          wn++;
        }
      }
    } else {
      if (yi <= y) {
        if (isLeft([xj, yj], [xi, yi], [x, y]) < 0) {
          wn--;
        }
      }
    }
  }
  return wn != 0;
};

function isLeft(P0, P1, P2) {
  let res = ( (P1[0] - P0[0]) * (P2[1] - P0[1])
            - (P2[0] -  P0[0]) * (P1[1] - P0[1]) );
  return res;
}