Unique objects from array with max property

max.js

arr = [
  { id: 324, difficulty: 3 },
  { id: 324, difficulty: 2 },
  { id: 324, difficulty: 5 },
  { id: 500, difficulty: 3 },
  { id: 500, difficulty: 3.5 },
  { id: 500, difficulty: 7 }
];

let map = _.groupBy(arr, (el) => el.id)
let array = [];
_.each(map, (element) => {
  let max = _.max(element, (el) => el.difficulty);
  console.log(max);
});