tee method
The tee() method of the
ReadableStream interface
tees the current
readable stream, returning a
two-element array containing the two resulting branches as
new ReadableStream instances.
This is useful for allowing two readers to read a stream sequentially or simultaneously, perhaps at different speeds. For example, you might do this in a ServiceWorker if you want to fetch a response from the server and stream it to the browser, but also stream it to the ServiceWorker cache. Since a response body cannot be consumed more than once, you'd need two copies to do this.
A teed stream will partially signal backpressure at the rate of the
faster consumer
of the two ReadableStream branches,
and unread data is enqueued internally on the slower consumed
ReadableStream
without any limit or backpressure.
That is, when both branches have an unread element in their internal
queue,
then the original ReadableStream's controller's internal queue will
start to fill up,
and once its ReadableStreamDefaultController.desiredSize ≤ 0
or byte stream controller ReadableByteStreamController.desiredSize ≤ 0,
then the controller will stop calling pull(controller) on the
underlying source passed to ReadableStream.ReadableStream.
If only one branch is consumed, then the entire body will be enqueued in
memory.
Therefore, you should not use the built-in tee() to read very large
streams
in parallel at different speeds.
Instead, search for an implementation that fully backpressures
to the speed of the slower consumed branch.
To cancel the stream you then need to cancel both resulting branches. Teeing a stream will generally lock it for the duration, preventing other readers from locking it.
Implementation
external JSArray<ReadableStream> tee();