ReadableStreamDefaultController.desiredSize

The desiredSize read-only property of the ReadableStreamDefaultController interface returns the desired size required to fill the stream's internal queue.

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

The A readable stream with an underlying push source and backpressure support example in the spec provides a good example of using desiredSize to manually detect when the stream is full and apply backpressure, and also of using ReadablestreamDefaultController.error() to manually trigger a stream error if another part of the system it relies on fails.