Design: Accessing the stack/heap from Javascript

Is it possible to access the heap of the calling WASM instance within an imported Javascript function?

Yes, you can access the linear memory if you export it from the module. It will be a WebAssembly.Memory object, and its buffer property is an ArrayBuffer with the contents of linear memory.

Related, is it possible to resume an instance after an imported Javascript function threw an exception?

No, that's not currently possible. The call stack is discarded when an exception is thrown. To do this, you'd have to instrument your module to allow for this behavior. It's likely that some of the extensions discussed in the exception-handling proposal would help here as well.

Is the heap contained in the memory buffer? I haven't seen it mentioned in the documentation.
It also seems impossible to access the call stack within the imported javascript function without instrumenting the WASM module.

A WebAssembly module doesn't have a heap, it just has a memory which is a flat vector of bytes. When a language like C++ is compiled to WebAssembly, the heap is stored in the memory, but so are other data.

In general, the WebAssembly call stack (which includes the stack of called functions and their locals) is not stored in memory, and is not accessible. But when a language like C++ is compiled to WebAssembly, some elements of the C++ stack may be stored in memory. Generally these are the "address-taken" C++ variables and large arrays.

Ah, I see, so that's what the __heap_base is for!

Instance {
  exports: 
   { memory: Memory {},
     __heap_base: 66560,
     __data_end: 1024,
     main: [Function: 2],
     fac: [Function: 3] }
}

Thank you very much!

WebAssembly.Memory is so called heap