test.js

import { pointInPolygon } from './utils';

const Vector = (x, y) => ({ x, y });

it('detects points in polygons', () => {
  const vertices = [
    [1, 1],
    [1, 4],
    [4, 4],
    [4, 1]
  ].map(([x, y]) => Vector(x, y));

  const polygon = { vertices };

  expect(pointInPolygon(Vector(2, 2), polygon)).toBeTruthy();
  //expect(pointInPolygon(Vector(1, 1), polygon)).toBeFalsy();
  //expect(pointInPolygon(Vector(1.5333, 2.3434), polygon)).toBeTruthy();
  //expect(pointInPolygon(Vector(400, -100), polygon)).toBeFalsy();
  //expect(pointInPolygon(Vector(1.01, 1.01), polygon)).toBeTruthy();
});

utils.js

const { min, max } = Math;
export const pointInPolygon = (p, polygon) => {
  let c = 0;
  let p1 = polygon.vertices[0];
  let p2;
  let n = polygon.vertices.length;
  let xinters;

  for (let i = 1; i <= n; i++) {
    p2 = polygon.vertices[i % n];
    if (p.y > min(p1.y, p2.y) &&
        p.y <= max(p1.y, p2.y) &&
        p.x <= max(p1.x, p2.x) &&
        p1.y !== p2.y) {
      xinters = (p.y - p1.y) * (p2.x - p1.x) / (p2.y - p1.y) + p1.x;
      if (p1.x === p2.x || p.x <= xinters) {
        c++;
      }
    }
    p1 = p2;
  }
  return c % 2 !== 0;
}