Nodejs Utililty Methods Array Group By

Array.prototype.groupBy = function(prop) {
  return this.reduce(function(groups, item) {
    var val = item[prop];
    groups[val] = groups[val] || [];
    groups[val].push(item);
    return groups;
  }, {});
let students = [
...

Array.prototype.groupBy = function (selector, comparer) {
    var grp = [];
    var l = this.length;
    comparer = comparer || EqualityComparer;
    selector = selector || Selector;
    for (var i = 0; i < l; i++) {
        var k = selector(this[i]);
        var g = grp.first(function (u) { return comparer(u.key, k); });
        if (!g) {
...