ReadableStream.ReadableStream()

Draft
This page is not complete.

This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The ReadableStream() constructor creates and returns a readable stream object from the given handlers.

Syntax

var readableStream = new ReadableStream(underlyingSource[, queueingStrategy]);

Parameters

underlyingSource
An object containing methods and properties that define how the constructed stream instance will behave. underlyingSource can contain the following:
start(controller)
This is a method, called immediately when the object is constructed. The contents of this method are defined by the developer, and should aim to get access to the stream source, and do anything else required to set up the stream fuctionality. If this process is to be done asynchronously, it can return a promise to signal success or failure. The controller parameter passed to this method is a ReadableStreamDefaultController or a ReadableByteStreamController, depending on the value of the type property. This can be used by the developer to control the stream during set up.
pull(controller) Optional
This method, also defined by the developer, will be called repeatedly when the stream's internal queue of chunks is not full, up until it reaches its high water mark. If pull() returns a promise, then it won't be called again until that promise fulfills; if the promise rejects, the stream will become errored. The controller parameter passed to this method is a ReadableStreamDefaultController or a ReadableByteStreamController, depending on the value of the type property. This can be used by the developer to control the stream as more chunks are fetched.
cancel(reason) Optional
This method, also defined by the developer, will be called if the app signals that the stream is to be cancelled (e.g. if ReadableStream.cancel() is called). The contents should do whatever is necessary to release access to the stream source. If this process is asynchronous, it can return a promise to signal success or failure. The reason parameter contains a DOMString describing why the stream was cancelled.
type Optional
This property controls what type of readable stream is being dealt with. If it is included with a value set to bytes, the passed controller object will be a ReadableByteStreamController capable of handling a BYOB (bring your own buffer)/byte stream. If it is not included, the passed controller will be a ReadableStreamDefaultController.
autoAllocateChunkSize Optional
For byte streams, the developer can set the autoAllocateChunkSize with a positive integer value to turn on the stream's auto-allocation feature. With this turned on, the stream implementation will automatically allocate an ArrayBuffer with a size of the given integer, and call the underlying source code as if the consumer was using a BYOB reader.
queueingStrategy Optional
An object that optionally defines a queueing strategy for the stream. This takes two parameters:
highWaterMark
A non-negative integer — this defines the total number of chunks that can be contained in the internal queue before backpressure is applied.
size(chunk)
A method containing a parameter chunk — this indicates the size to use for each chunk, in bytes.

Note: You could define your own custom queueingStrategy, or use an instance of ByteLengthQueueingStrategy or CountQueueingStrategy for this object value. If no queueingStrategy is supplied, the default used is the same as a CountQueuingStrategy with a high water mark of 1.

Return value

An instance of the ReadableStream object.

Exceptions

RangeError
The supplied type value is neither bytes nor undefined.

Examples

TBD

Specifications

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

Browser compatibility

No compatibility data found. Please contribute data for "path.to.feature.NameOfTheConstructor" (depth: 1) to the MDN compatibility data repository.

Document Tags and Contributors

 Contributors to this page: chrisdavidmills
 Last updated by: chrisdavidmills,