Most userland implementations of custom ReadableStream instances do not typically bother with all the ceremony required to correctly implement both default and BYOB read support in a single stream – and for good reason. It's difficult to get right and most of the time consuming code is typically going to fallback on the default read path. The example below shows what a "correct" implementation would need to do. It's big, complex, and error prone, and not a level of complexity that the typical developer really wants to have to deal with:
The Web streams spec requires promise creation at numerous points, often in hot paths and often invisible to users. Each read() call doesn't just return a promise; internally, the implementation creates additional promises for queue management, pull() coordination, and backpressure signaling.
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