ReadableStreamDefaultController

The ReadableStreamDefaultController interface of the Streams API represents a controller allowing control of a ReadableStream's state and internal queue. Default controllers are for streams that are not byte streams.

Constructor

ReadableStreamDefaultController()

Creates and returns a ReadableStreamDefaultController() object instance.

Properties

ReadableStreamDefaultController.desiredSize

Returns the desired size required to fill the stream's internal queue.

Methods

ReadableStreamDefaultController.close()

Closes the associated stream.

ReadableStreamDefaultController.enqueue()

Enqueues a given chunk in the associated stream.

ReadableStreamDefaultController.error()

Causes any future interactions with the associated stream to error.

Examples

In the following simple example, a custom ReadableStream is created using a constructor (see our Simple random stream example for the full code). The start() function generates a random string of text every second and enqueues it into the stream. A cancel() function is also provided to stop the generation if ReadableStream.cancel() is called for any reason.

Note that a ReadableStreamDefaultController object is provided as the parameter of the start() and pull() functions.

When a button is pressed, the generation is stopped, the stream is closed using ReadableStreamDefaultController.close(), and another function is run, which reads the data back out of the stream.

const stream = new ReadableStream({
  start(controller) {
    interval = setInterval(() => {
      let string = randomChars();

      // Add the string to the stream
      controller.enqueue(string);

      // show it on the screen
      let listItem = document.createElement('li');
      listItem.textContent = string;
      list1.appendChild(listItem);
    }, 1000);

    button.addEventListener('click', function() {
      clearInterval(interval);
      fetchStream();
      controller.close();
    })
  },
  pull(controller) {
    // We don't really need a pull in this example
  },
  cancel() {
    // This is called if the reader cancels,
    // so we should stop generating strings
    clearInterval(interval);
  }
});

Specifications

Browser Compatibility

[1] Readable streams are currently enabled in Firefox, but hidden behind the dom.streams.enabled and javascript.options.streams prefs.