index.js

const records = [
  { id: 'a', category: 1 },
  { id: 'b', category: 2 },
  { id: 'c', category: 3 },
  { id: 'd', category: 1 },
  { id: 'e', category: 2 },
  { id: 'f', category: 3 },
  { id: 'g', category: 1 },
  { id: 'h', category: 2 }
];
const result = records
  .sort((a, b) => a.category - b.category)
  .map(r => ({
    key: r.category,
    value: {
      id: r.id,
      category: r.category
    }
  }));

console.log(result);