ReadableStreamDefaultReader.cancel()

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

The cancel() method of the ReadableStreamDefaultReader interface cancels the stream, signaling a loss of interest in the stream by a consumer. The supplied reason argument will be given to the underlying source, which may or may not use it.

Cancel is used when you've completely finished with the stream and don't need any more data from it, even if there are chunks enqueued waiting to be read. That data is lost after cancel is called, and the stream is not readable any more. To read those chunks still and not completely get rid of the stream, you'd use ReadableStreamDefaultController.close().

Note: If the reader is active, the cancel() method behaves the same as that for the associated stream (ReadableStream.cancel()).

Syntax

Promise<reason> readableStreamDefaultReaderInstance.cancel(reason);

Parameters

reason Optional
A DOMString providing a human-readable reason for the cancellation.

Return value

A Promise, which fulfills with the value given in the reason parameter.

Exceptions

TypeError
The source object is not a ReadableStreamDefaultReader, or the stream has no owner.

Examples

In the following simple example, a previously-created custom ReadableStream is read using a ReadableStreamDefaultReader created using getReader(). (this code is based on our Simple random stream example). Each chunk is read sequentially and output to the UI, until the stream has finished being read, at which point we return out of the recursive function and print the entire stream to another part of the UI.

When the stream is done (if (done)), we run reader.cancel() to cancel the stream, signalling that we don't need to use it any more.

function fetchStream() {
  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");
      reader.cancel();
      para.textContent = result;
      return;
    }

    // value for fetch streams is a Uint8Array
    charsReceived += value.length;
    const chunk = value;
    let listItem = document.createElement('li');
    listItem.textContent = 'Received ' + charsReceived + ' characters so far. Current chunk = ' + chunk;
    list2.appendChild(listItem);

    result += chunk;

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

Specifications

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

Browser compatibility

No compatibility data found. Please contribute data for "path.to.feature.NameOfTheProperty" (depth: 1) to the MDN compatibility data repository.

Document Tags and Contributors

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