getters.spec.js

// the getter
getElemsByCategory: state => (category) => {
  return state.elems.filter((el, index, arr) => el.category === category)
}

// the test
it('should return the elements that have Cat1 as category', done => {
  const expected = [
    {
      'id': '1',
      'category': 'Cat1'
    },
    {
      'id': '2',
      'category': 'Cat1'
    }
  ]
  
  const state = [
    {
      'id': '1',
      'category': 'Cat1'
    },
    {
      'id': '2',
      'category': 'Cat1'
    },
    {
      'id': '3',
      'category': 'Cat2'
    }
  ]

  expect(getters.getElemsByCategory(state)('Cat1')).to.be.deep.equal(expected)

  done()
})