ReadableStream: 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 desiredSize ≤ 0 or byte stream controller desiredSize ≤ 0, then the controller will stop calling pull(controller) on the underlying source passed to 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.

tee()

function teeStream() {
  const teedOff = stream.tee();
  fetchStream(teedOff[0], list2);
  fetchStream(teedOff[1], list3);
}

function fetchStream(stream, list) {
  const reader = stream.getReader();
  let charsReceived = 0;

  // read() returns a promise that resolves
  // when a value has been received
  reader.read().then(function processText({ done, value }) {
    // Result objects contain two properties:
    // done  - true if the stream has already given you all its data.
    // value - some data. Always undefined when done is true.
    if (done) {
      console.log("Stream complete");
      return;
    }

    // value for fetch streams is a Uint8Array
    charsReceived += value.length;
    const chunk = value;
    let listItem = document.createElement("li");
    listItem.textContent = `Read ${charsReceived} characters so far. Current chunk = ${chunk}`;
    list.appendChild(listItem);

    // Read some more, and call this function again
    return reader.read().then(processText);
  });
}