withinBox.js

const box = {
  sw: {long: -16.14990234375, lat: -26.980828590472107}, // bl
  ne: {long: 39.7265625, lat: 20.632784250388028}, // tr
};

const points = [
  {long: 21.62109375, lat: 3.601142320158735}, // inside
  {long: -11.513671874999998, lat: 16.551961721972525}, // inside
  {long: 28.4765625, lat: -19.228176737766248}, // inside
  {long: 4.130859375, lat: 0.4394488164139768}, // inside
  {long: 13.359375, lat: 26.115985925333536}, // outside
  {long: 46.7578125, lat: -4.039617826768424}, // outside
  {long: 11.25, lat: -27.293689224852393}, // outside
  {long: -19.335937499999996, lat: 3.0746950723696944}, // outside
];

const withinBox = function(sw, ne, point) {
  return point.lat >= sw.lat && point.lat <= ne.lat 
     && (ne.long < sw.long ? (point.long >= sw.long || point.long <= ne.long) : (point.long >= sw.long && point.long <= ne.long));
};

const inside = points.filter(point => {
  return withinBox(box.sw, box.ne, point);
});

console.log(inside);