---
hide_title: false
hide_table_of_contents: false
pagination_next: null
pagination_prev: null
---

# desiredSize

The **`desiredSize`** read-only property of the`ReadableByteStreamController` interface returns the number of bytes required to fill the stream's internal queue to its "desired size".

The value is used by the stream to indicate a preferred flow rate to the underlying source.
Sources that support throttling or pausing their inflow of data (not all do!) should control the inflow such that `desiredSize` of the stream buffer is kept positive and as close to zero as possible.

The `desiredSize` is used to apply backpressure from downstream consumers.

## Value

An integer. Note that this can be negative if the queue is over-full.

The value will be `null` if the stream has errored and `0` if it is closed.
