Potentially a better way to design streaming byte storage.

byteStorage.js

// Byte Storage as Content Addressable Storage

class ByteStorage {
  async set (key, value) async {
    // Send to temporary storage w/ random UUID
    let tmpfile = randomFile()
    let p1 = send(value, tmpfile)
    // Also stream the value to a hasher
    let p2 = send(value, /* streaming hashing function */ )
    let values = await Promise.all(p1, p2)
    moveFile(tmpfile, values[1])
    return values[1]
  }
  async get (key) {
    return readFileFilesystem(key)
  }
}

// Building a higher order store w/ human readable names on top.

class FriendlyStore {
  constructor () {
    this.byteStorage = new ByteStorage()
    this.idb = new IDB() // IDB w/ promises
  }
  async set (key, value) {
    let oldkey = await this.idb.get(key)
    let newkey = await this.byteStorage.set(value)
    if (oldkey === null || oldkey === newkey) {
      return await this.idb.set(key, newkey)
    } else {
      throw new Error('Update Conflict')
    }
  }
  async get (key) {
    let hash = await this.idb.get(key)
    return this.byteStorage.get(hash)
  }
}

A complete implementation of a content addressable store on the filesystem is here: https://github.com/mikeal/lucass/blob/master/fs.js although it doesn't use promises :)