javascript underscoreを使ってオブジェクトの配列をプロパティ値でグループ分けする

<script src="https://cdn.jsdelivr.net/npm/underscore@1.13.4/underscore-umd-min.js"></script>

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 = _.indexBy(arr, 'age');

console.log( result );