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 compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
Basic support
Experimental
Chrome Full support 43Edge Full support 16Firefox Full support 57
Disabled
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 NoOpera Full support 30Safari ? WebView Android Full support 43Chrome Android Full support 43Edge Mobile Full support 16Firefox Android Full support 57
Disabled
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 30Safari iOS ? Samsung Internet Android Full support 4.0

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
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.

Document Tags and Contributors

Contributors to this page: Septatrix, fscholz, chrisdavidmills
Last updated by: Septatrix,