html-cache.js

const stream = require('stream')
const cache = new Map() // you might wanna use an lru here

function createCacheStream (url) {
  const buf = []
  return stream.Transform({
    transform: function (data, enc, cb) {
      buffer.push(data)
      cb(null, data)
    },
    flush: function (cb) {
      cache.set(url, Buffer.concat(buf))
      cb()
    }
  })
}