> Does anybody know why is it such a big problem to add dom access to wasm?
Well, the article does a pretty good job of answering this specific question ;)
Well, the article does a pretty good job of answering this specific question ;)
It gives good reasons why we can't have specific parts. Having the JavaScript Standard library in WebAssembly would be hard (was anyone actually asking for that?), and some of the modern APIs using promises or iterators wouldn't have a clear mapping. Also not everything could be zero-copy for every language
But the article doesn't do a very good job explaining why we can't have some dom access, at least for the 90% of DOM APIs not using JavaScript-specific features.
Most of the argument boils down to "you shouldn't want direct DOM access, because doing that would be work for these people, and we can instead have those other people do lots of work making the JavaScript bridge less painful. And anyways it's not clear if having these people make proper APIs would actually result in faster code than having those people do a separate sophisticated toolchain for each language"
It reads very much like a people and resource allocation problem rather than a technical challenge
The DOM is a Javascript API, so it uses 100% Javascript-specific features (every DOM manipulation requires accessing JS objects and their properties and lots of those properties and function args are Javascript strings) - none of those map trivially to WASM concepts.
It's a bit like like asking why x86 assembly code doesn't allow "C++ stdlib access", the question doesn't even make much sense ;)
Or is there something in the browser architecture that requires them to be JavaScript objects with the memory layout of the JavaScript engine, rather than just conceptually being objects?
On the one hand, JS DOM objects are idl generated wrappers for C++. In theory we can generate more WASM friendly wrappers.
On the other, the C++ code implementing the API will be tightly coupled to the entire JS type system and runtime. Not just the concept of an object but every single design decision from primitives to generators to dynamic types to prototypical inheritance to error handling...
Also, I believe the C++ DOM implementation itself is pretty tightly integrated with javascript and it's memory management e.g. nodes have references into the managed heap to use JS objects directly like EventListeners and js functions.
Creating a new non-JS DOM API doesn't sound intractable to me... but browsers annihilate my assumptions so it's probably millions of hours of effort and close to a rewrite...
Maybe I misunderstood, but isn't Dom access in essence an ability to change html tree? Since this is wasm, why would it need to reimplement js API and need type mappings? Couldn't it be something different?
It doesn't need to be the same API... but implementing a new DOM API that doesn't meet the w3c standard is a bit on the nose. It's meant to be language independent hence the IDL.
Looking into it, the IDL might insulate the existing API implementation from JS to a greater degree than I assumed above. Apparently there might be horrors lurking in the binding generation code though. You can poke around in blink:
https://github.com/chromium/chromium/blob/main/third_party/b...
> isn't Dom access in essence an ability to change html tree
It might be more accurate to look at it the other way: our current DOM implementations were created to implement the "DOM API Standard". The standard dictated the types and how reading/mutation works.
> need type mappings
I can't imagine how it can avoid type mappings for things like e.g. creating unattached bits of DOM or binding callbacks that receive bits of DOM.
Personally I might be happy for a tiny WASM api... but then foresee 10 years of maddening omissions, security bugs and endless moaning because they didn't just implement the standard everyone already knew.
This in the same conversation where people are saying the DOM API is intrinsically tied to Javascript!
The underlying problem is that you need to translate between a Javascript-idiomatic API and whatever is an idiomatic API in the language you compile into WASM, and idiomatic C, C++, Rust, ... APIs all look very different so there isn't a single answer. It's not WASM being relevant here, but the high level source language that's compiled into WASM, and how well the DOM JS API (and the Javascript concept it is built on) map to APIs for various 'authoring languages'.
The whole problem is really quite similar to connecting two higher level languages (e.g. Rust and C++) through a C API (since this is the common interface that both Rust and C++ can talk to - but that doesn't mean that you can simply send a Rust String to the C++ side and expect that you automatically get a std::string - this sort of magic needs to be explicitly implemented in a shim layer sitting between the two worlds) - and to use the string example, there is no such thing as a WASM string, instead how string data is represented on the WASM side depends on the source language that's compiled to WASM.
Of course you couldn't provide idiomatic versions for every language, but the JS shims also can't really do that. Providing something close to idiomatic C would be a huge step up, the language libraries can then either offer a C-like API or choose to build new abstractions on top of it
That's easier said than done because of details like that you can't build a JS string on the C++ side, the translation from string data on the WASM heap into a JS object needs to happen on the JS side.
But this is how all the Emscripten web API shims work, and they do this quite efficiently, and some of those shims also split their work between the JS and C/C++ side.
So to the programmer it does look like with Emscripten there's direct access to the (for instance) WebGL or WebGPU APIs, but in reality there's still quite a bit of JS code involved for each call (which isn't a problem really as long as no expensive marshalling needs to happen in such a call, since the call overhead from WASM into JS alone is really minimal).