ReadableStreamDefaultController.ReadableStreamDefaultController()

The ReadableStreamDefaultController() constructor creates and returns a ReadableStreamDefaultController object instance.

Note: You wouldn't use this constructor manually — this is used during the construction of a ReadableStream object.

Syntax

var readableStreamDefaultController = new ReadableStreamDefaultController(stream, underlyingSource, size, highWaterMark);

Parameters

stream
The ReadableStream to be controlled.
underlyingSource
An object containing methods and properties that define how the constructed stream instance will behave. See the ReadableStream() constructor's parameters definitions for more information.
size
A method containing a parameter chunk — this indicates the size to use for each chunk, in bytes.
highWaterMark
A non-negative integer — this defines the total number of chunks that can be contained in the internal queue before backpressure is applied.

Return value

An instance of the ReadableStreamDefaultController object.

Exceptions

TypeError
The supplied stream parameter is not a ReadableStream, or it already has an associated controller.

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

Specification Status Comment
Streams
The definition of 'ReadableStreamDefaultController()' in that specification.
Living Standard Initial definition.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
ReadableStreamDefaultController() constructor
Experimental
Chrome ? Edge ? Firefox Full support 65
Full support 65
Full support 57
Disabled
Disabled From version 57: this feature is behind the dom.streams.enabled preference (needs to be set to true) and the javascript.options.streams preference (needs to be set to true). To change preferences in Firefox, visit about:config.
IE ? Opera ? Safari ? WebView Android ? Chrome Android ? Firefox Android Full support 65
Full support 65
Full support 57
Disabled
Disabled From version 57: this feature is behind the dom.streams.enabled preference (needs to be set to true) and the javascript.options.streams preference (needs to be set to true). To change preferences in Firefox, visit about:config.
Opera Android ? Safari iOS ? Samsung Internet Android ?

Legend

Full support  
Full support
Compatibility unknown  
Compatibility unknown
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.
User must explicitly enable this feature.
User must explicitly enable this feature.