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 simultaneously, perhaps at different speeds. You might do this for example 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.

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.


var teedStreams = readableStream.tee();



Return value

An Array containing two ReadableStream instances.


The source stream is not a ReadableStream.


In the following simple example, a previously-created stream is teed, then both resulting streams (contained in two members of a generated array) are passed to a function that reads the data out of the two streams and prints each stream's chunks sequentially to a different part of the UI. See Simple tee example for the full code.

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 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");

    // 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;

    // Read some more, and call this function again


Specification Status Comment
The definition of 'tee()' in that specification.
Living Standard Initial definition.

Browser compatibility

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