Web/API/ReadableStream/tee

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.

Syntax

var teedStreams = readableStream.tee();

Parameters

None.

Return value

An Array containing two ReadableStream instances.

Exceptions

TypeError: The source stream is not a ReadableStream.

Examples

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 [[../../../../../../../mdn.github.io/dom-examples/streams/simple-tee-example/index|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
  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);
  });
}

Specifications

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

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`tee` Experimental'	Chrome Full support 43	Edge Full support 79	Firefox Full support 65 Full support 65 Full support 57 Disabled' Disabled' From version 57: this feature is behind the `dom.streams.enabled` preference (needs to be set to `true`) and the `javascript.options.streams` preference (needs to be set to `true`). To change preferences in Firefox, visit about:config.	IE No support No	Opera Full support 30	Safari Full support 10.1	WebView Android Full support 43	Chrome Android Full support 43	Firefox Android Full support 65 Full support 65 Full support 57 Disabled' Disabled' From version 57: this feature is behind the `dom.streams.enabled` preference (needs to be set to `true`) and the `javascript.options.streams` preference (needs to be set to `true`). To change preferences in Firefox, visit about:config.	Opera Android Full support 30	Safari iOS Full support 10.3	Samsung Internet Android Full support 4.0

Legend

Full support: Full support
No support: No support
Experimental. Expect behavior to change in the future.': Experimental. Expect behavior to change in the future.
User must explicitly enable this feature.': User must explicitly enable this feature.

ReadableStream.tee() by Mozilla Contributors is licensed under CC-BY-SA 2.5.