Adds an onProgress mechanism to track Download Progress when using the `fetch` API.

progressHelper.js

function progressHelper(onProgress) {
  return (response) => {
    if (!response.body) return response

    let loaded = 0
    const contentLength = response.headers.get('content-length')
    const total = !contentLength ? -1 : parseInt(contentLength, 10)

    return new Response(
      new ReadableStream({
        start(controller) {
          const reader = response.body.getReader()
          return read()

          function read() {
            return reader.read()
              .then(({ done, value }) => {
                if (done) return void controller.close()
                loaded += value.byteLength
                onProgress({ loaded, total })
                controller.enqueue(value)
                return read()
              })
              .catch(error => {
                console.error(error)
                controller.error(error)
              })
          }
        }
      })
    )
  }
}