How to use the stopword.en function in stopword

/* global it */
/* global describe */

const Readable = require('stream').Readable
const logLevel = process.env.LOG_LEVEL || 'error'
const s = new Readable({ objectMode: true })
const sandboxPath = 'test/sandbox'
const should = require('should')
const stopwords = require('stopword').en

s.push({
  id: 'a',
  title: 'The Beatles',
  content: 'The Beatles were an English rock band, formed in Liverpool in 1960. Beatles from Liverpool',
  year: ['1960', '1961', '1962']
})
s.push({
  id: 'b',
  title: 'The Rolling Stones',
  content: 'The Rolling Stones are an English rock band formed in London in 1962.',
  year: ['1962', '1963', '1964']
})
s.push({
  id: 'c',
  title: 'Pink Floyd',

import stopword from 'stopword'

import pipeline from './search-index-pipeline'
import { convertMetaDocId } from 'src/activity-logger'
import { RESULT_TYPES } from 'src/overview/constants'

const indexOpts = {
    batchSize: 500,
    appendOnly: false,
    indexPath: 'worldbrain-index',
    logLevel: 'info',
    preserveCase: false,
    compositeField: false,
    nGramLength: 1,
    // separator: /[|' .,\-|(\n)]+/,
    stopwords: stopword.en,
    fieldOptions: {
        // The `domain.tld(.cctld)` data from a page's URL
        // Currently used to afford `domain.tld(.cctld)` search in our queries
        // Should never need to tokenize, but put forward-slash separator incase preproecssing fails for whatever reason
        // (then domain search can still happen)
        domain: {
            weight: 40,
            fieldedSearch: true,
            separator: '/',
        },
        // Page title text; occasionally empty
        title: {
            weight: 30,
            fieldedSearch: true,
        },
        // Page URL tokenized by forward slashes; normalized slightly to remove protocol and leading `www.`