ReadableStreamDefaultController: enqueue() method

chunk

The chunk to enqueue.

None (undefined).

TypeError

Thrown if enqueue() is called when the stream is not readable — because it is already closed, cancelled, or errored — or because it has been requested to close by the underlying source but it has not yet done so because there are still enqueued chunks to read.

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 — see controller.enqueue(string). A cancel() function is also provided to stop the generation if ReadableStream.cancel() is called for any reason.

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.

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);
  },
});

• Using readable streams
• ReadableStreamDefaultController