web api interface
ReadableStreamDefaultController
Available in 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
desiredSizeRead only- : Returns the desired size required to fill the stream’s internal queue.
Instance methods
close()- : Closes the associated stream.
enqueue()- : Enqueues a given chunk in the associated stream.
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 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 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);
},
});