mapfilter function, just like map, but can also discard elements. Equivalent to array.map(fun).filter(n => n !== undefined)

mapfilter.js

function mapfilter(array, fun) {
  // Equivalent to array.map(fun).filter(x => x !== undefined)
  var res = [];
  for(var i=0; i<array.length; i++) {
    var r = fun(array[i], i, array);
    if (r !== void 0) res.push(r);
  }
  return res;
}

Exemple use :

mapfilter([-10, 0, 100], n => n<0 ? undefined : Math.sqrt(n)) // [0, 10]