FFMPEG Get FileStream rather than writing straight to a file about node-ytdl-core

[TimeForANinja]

const ytdl = require('ytdl-core');
const ref = 'https://www.youtube.com/watch?v=aqz-KE-bpKQ';
// Get audio and video streams
const audio = ytdl(ref, { quality: 'highestaudio' })
const video = ytdl(ref, { quality: 'highestvideo' })

[UtterDonkey]

I want to combine the audio and video streams using FFMPEG, like in the FFMPEG example, but get a readable stream instead of FFMPEG writing it straight to a file.