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# TransformStreamDefaultController.desiredSize

The **`desiredSize`** read-only property of the `TransformStreamDefaultController` interface returns the desired size to fill the queue of the associated [`ReadableStream`](../../../globals/ReadableStream/ReadableStream.mdx).

The internal queue of a `ReadableStream` contains chunks that have been enqueued, but not yet read.

If the `desiredSize` is `0` then the queue is full. Therefore you can use this information to manually apply backpressure to manage the queue.

## Value

The desired size.
