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
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);
},
});
Specifications
Specification |
---|
Streams Standard # rs-default-controller-class |
Browser compatibility
BCD tables only load in the browser
See also
- Streams API concepts
- Using readable streams
ReadableStream
- WHATWG Stream Visualizer, for a basic visualization of readable, writable, and transform streams.
- Web-streams-polyfill or sd-streams - polyfills