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ReadableStream.tee

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This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

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

readableStreamInstance.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 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
Streams
The definition of 'tee()' in that specification.
Living Standard Initial definition.

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
Basic support 43 16 57
Disabled
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.
No 30 ?
Mobile
Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android iOS Safari Samsung Internet
Basic support 43 43 16 57
Disabled
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.
30 ? 4.0

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/tee