sample_data.js

export default {
  data: [
    {
      id: 1,
      level: 1,
      parent_id: null,
      children: [ 4,5 ]
    },
    {
      id: 2,
      level: 1,
      parent_id: null,
      children: [ 6 ]
    },
    {
      id: 3,
      level: 1,
      parent_id: null,
      children: []
    },
    {
      id: 4,
      level: 2,
      parent_id: 1,
      children: []
    },
    {
      id: 5,
      level: 2,
      parent_id: 1,
      children: []
    },
    {
      id: 6,
      level: 2,
      parent_id: 2,
      children: []
    }
  ]
}

sample_store.js

import sample_data from './sample_data';

const state = Object.assign({}, sample_data);
const getters = {
  getLevel: state => level => state.data.filter(item => item.level === level) 
}

export default {
  namespaced: true,
  state,
  getters
}