FWIW Python is also a great choice for composability. gevent is the only widely used library that really monkey-patches, and with it (to answer the question in OP) you can write essentially any function as a pseudo-goroutine (greenlets that yield to the event loop when reading from a queue which allows you to apply backpressure). And you can still access the vast realm of Python libraries; any that use sockets will automatically yield to the event loop on blocking operations. The criticisms from the recent Python complaint thread are valid, but it's still a great language for using other code.

For JS, rarely do people mutate Object.prototype these days; everything's a functional library. So almost all libraries you use will be good actors. It's also a good choice, though for data management and interop with scientific/vector/tensor operations, it's still hard to beat Python.