E.memoryArea not working on ESP8266 (due to char-iterator ?)

Interesting tests, but a bit difficult to follow without any code snippets. For example, I don't know what you mean by "This time the SHA1 was copied to flash without function body." If you didn't put the SHA1 function body in flash, then what did you put in flash?

I can see how to execute code from a memory area, what I'm not yet seeing is how to put it all together. One option could be to have a special method to load a bunch of source code into a flash area and tell the interpreter: here, load this. But that now creates a completely separate path from the way the IDE or command line tool work to load code.

Or we could modify some stuff in jsinteractive that puts code into a memory area by default. But now we need to create a memory allocator for the flash area that can also deal with code being replaced, i.e. areas being freed, etc. On top of that then comes save() which somehow needs to make sure that whatever is in RAM that points to the flash is saved (perhaps that's in a JSvar so automatically gets saved, I haven't looked). There's probably a simple solution that goes 90% of the way, like dropping the flash area on reset() and only adding to it, never freeing until the next reset().