ReadableStreamDefaultController

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.

Note: This feature is available in Web Workers.

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

None. ReadableStreamDefaultController instances are created automatically during ReadableStream construction.

Instance properties

ReadableStreamDefaultController.desiredSize Read only

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

Instance 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.

js
let interval;
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", () => {
      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);
  },
});