The fact that streamifyResponse and HttpResponseStream are only available embedded in the runtime makes it difficult to write simple, coherent unit tests for streaming lambda handlers. There's been at least one good attempt to create a wrapper to support easy testing, but it's not completely interface compatible (lacking, for example, the HttpResponseStream.from(underlyingStream, prelude) method) and it seems wasteful to rewrite/repackage code that's already written just to unbundle it from the rest of the NodeJS runtime.