lazy loaded `TransformStream` polyfill is not supported

[JounQin]

As title.

import * as streamSaver from 'streamsaver'
// for proper tree shaking
import { TransformStream } from 'web-streams-polyfill/ponyfill'

export const download = async () => {
  if (!window.TransformStream) {
    window.TransformStream = TransformStream
  }

  // `streamSaver` related codes
}

And streamSaver.TransformStream is not configurable nor writable, so it can not be changed dynamically like streamSaver.WritableStream.

[jimmywarting]

StreamSavers don't work with TransformStreams unless they are natively supported and if ReadableStream are transferable with postMessage.

• First of all polyfilled streams are not able to be transfered with postMessages, so that is why it's tested first and formost:

  test(() => {
    // Transferable stream was first enabled in chrome v73 behind a flag
    const { readable } = new TransformStream()
    const mc = new MessageChannel()
    mc.port1.postMessage(readable, [readable])
    mc.port1.close()
    mc.port2.close()
    supportsTransferable = true
    // Freeze TransformStream object (can only work with native)
    Object.defineProperty(streamSaver, 'TransformStream', {
      configurable: false,
      writable: false,
      value: TransformStream
    })
  })

• Secondly it freezes the TransformStream if to the native implementation if transferable streams are supported cuz the native implementation is preferred over everything else.
• If transferable streams is not supported at all, then there is no point in using TransformStream so StreamSaver just resolves into using only WritableStreams that pipes the data to MessageChannel and then forwards it to the service worker

so even if you assign TransformStream to the global scope before importing StreamSaver then it has no use for it anyway.

[JounQin]

@jimmywarting

The documentation seems incorrect.

[jimmywarting]

StreamSaver operates on two modes:

• Either it creates a Readable + Writable stream (using TransformStream), transfers the readable to service worker and hands the Writable to the developer
• If transferable stream isn't supported then it falls back to using only WritableStream and sending the data over DataChannel and tries to ping the service worker every x sec to keep service worker alive

[jimmywarting]

Ups :P

Setting TransformStream should have no affect :P should remove it from ReadMe ^^

[JounQin]

Assigning WritableStream seems should be:

streamSaver.WritableStream = ponyfill.WritableStream

[jimmywarting]

Yep, that should stay

[jimmywarting]

Maybe

streamSaver.WritableStream = streamSaver.WritableStream || ponyfill.WritableStream

[JounQin]

Working on a PR.