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# ReadableStream()

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

## Syntax

```js
new ReadableStream()
new ReadableStream(underlyingSource)
new ReadableStream(underlyingSource, queuingStrategy)
```

### Parameters

- `underlyingSource` _**optional**_

  - : An object containing methods and properties that define how the constructed stream instance will behave.
    `underlyingSource` can contain the following:

    - `start` (controller) _**optional**_
      - : 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
        functionality. 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
        string 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 is set, the stream implementation will automatically allocate a view buffer of the specified size in `ReadableByteStreamController.byobRequest` when required.

        This must be set to enable zero-copy transfers to be used with a default `ReadableStreamDefaultReader`.
        If not set, a default reader will still stream data, but `ReadableByteStreamController.byobRequest` will always be `null` and transfers to the consumer must be via the stream's internal queues.

- `queuingStrategy` _**optional**_

  - : An object that optionally defines a queuing 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
    > `queuingStrategy`, or use an instance of
    > `ByteLengthQueuingStrategy` or `CountQueuingStrategy`
    > for this object value. If no `queuingStrategy` 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`
  - Thrown if the supplied type value is neither `"bytes"` nor `undefined`.
