how to go functional?

test.js

'use strict';

const _ = require('lodash');
const chai = require('chai');
const should = chai.should();

describe('filter groups behavior', () => {
  const X = [
    {name: 'a'},
    {name: 'b'},
    {name: 'c'},
    {name: 'd'},
    {name: 'e'},
    {name: 'f'},
  ];

  const Y = [
    {name: 'b'},
    {name: 'c'},
    {name: 'd'}
  ];

  const Z = ['a', 'b', 'e', 'g'];

  describe('the original way', () => {
    function groupsToAdd(originGroups, groupsToFilter, groupsToKeep) {
      const groups = _.filter(originGroups, (elt) => _.findIndex(groupsToFilter, {name: elt.name}) < 0);
      return _.filter(groups, (elt) => _.indexOf(groupsToKeep, elt.name) > -1);
    }

    function groupsToDelete(originGroups, groupsToFilter) {
      return _.filter(originGroups, (elt) => _.indexOf(groupsToFilter, elt.name) < 0);
    }

    it('should keep groups in X that are in Z but not in Y', () => {
      const groups = groupsToAdd(X, Y, Z);

      groups.should.have.a.lengthOf(2);
      groups[0].name.should.equal('a');
      groups[1].name.should.equal('e');
    });

    it('should keep groups in X that are not in Z', () => {
      const groups = groupsToDelete(X, Z);

      groups.should.have.a.lengthOf(3);
      groups[0].name.should.equal('c');
      groups[1].name.should.equal('d');
      groups[2].name.should.equal('f');
    });
  });

  describe('the functional way', () => {
    const filter = (fn) => (array) => _.filter(array, fn);
    const existsIn = (array) => (fn) => (element) =>_.indexOf(array, fn(element)) > -1;
    const doesNotExistsIn = (array) => (fn) => (element) => _.indexOf(array, fn(element)) < 0;
    const getName = o => o.name;
    const map = (array) => (fn) => array.map(fn);

    function groupsToAdd(originGroups, groupsToFilter, groupsToKeep) {
      const firstFilter = filter(doesNotExistsIn(map(groupsToFilter)(getName))(getName));
      const secondFilter = filter(existsIn(groupsToKeep)(getName));
      return secondFilter(firstFilter(originGroups));
    }

    function groupsToDelete(originGroups, groupsToFilter) {
      return filter(doesNotExistsIn(groupsToFilter)(e => e.name))(originGroups);
    }

    it('should keep groups in X that are in Z but not in Y', () => {
      const groups = groupsToAdd(X, Y, Z);

      groups.should.have.a.lengthOf(2);
      groups[0].name.should.equal('a');
      groups[1].name.should.equal('e');
    });

    it('should keep groups in X that are not in Z', () => {
      const groups = groupsToDelete(X, Z);

      groups.should.have.a.lengthOf(3);
      groups[0].name.should.equal('c');
      groups[1].name.should.equal('d');
      groups[2].name.should.equal('f');
    });
  });
});