groupByYear.js

{
    1991: [
      {
        {
          artist: 'Pearl Jam',
          album: 'Ten',
          year: '1991',
        },
        {
          artist: 'Soundgarden',
          album: 'Badmotorfinger',
          year: '1991',
        },
      }
    ],
    1994: [
      {
        artist: 'Soundgarden',
        album: 'Superunknown',
        year: '1994',
      },
    ],
    1998: [
      {
        artist: 'Pearl Jam',
        album: 'Yield',
        year: '1998',
      },
    ]
  }