JavaScript array extension

Array_extension.js

//////////////////////////
Object.defineProperty(Array.prototype, 'group', {
  enumerable: false,
  value: function (key) {
    var map = {};
    this.forEach(function (e) {
      var k = key(e);
      map[k] = map[k] || [];
      map[k].push(e);
    })
    return Object.keys(map).map(function (k) {
      return { key: k, data: map[k] };
    })
  }
})
//////////////////////////
Object.defineProperty(Array.prototype, 'group2', {
  enumerable: false,
  value: function (key) {
    var map = {};
    this.map(e => ({ k: key(e), d: e })).forEach(e => {
      map[e.k] = map[e.k] || [];
      map[e.k].push(e.d);
    });
    return Object.keys(map).map(k => ({ key: k, data: map[k] }));
  }
});