javascript オブジェクトの配列からプロパティでgroup byする

const arr = [
  { name: 'taro', age: 20 },
  { name: 'jiro', age: 20 },
  { name: 'saburo', age: 30 },
  { name: 'siro', age: 20 },
  { name: 'goro', age: 35 },
  { name: 'rokuro', age: 35 }
];

const result = arr.reduce((group, p) => {  

  group[p.age] = group[p.age] ?? [];
  
  group[p.age].push(p);

  return group;

}, {});

console.log( result );