direct_proxies "problem"

Tom Van Cutsem tomvc.be at gmail.com
Sat Jan 12 07:46:51 PST 2013


2013/1/11 Nathan Wall <nathan.wall at live.com>

> I thought part of the goal in ES6 was striving to make the DOM emulable
> (dare I say implementable?) in pure ES. If that's the case, the inability
> of DOM objects to be proxied sounds like a big deal.
>

I think we are missing an important point in this whole "Proxy the DOM"
discussion.

The way I have always thought of how "emulating the DOM using proxies"
would work is as follows:

1) a DOM-in-JS library (like dom.js) exports an API that is an exact mirror
copy of the DOM API.
2) some of these exported methods may create and return proxies that
emulate particular DOM objects.
3) application code that uses this library only ever interacts with these
emulated DOM objects *via the DOM emulation library's API*. In other words,
it *never* would try to pass an emulated DOM node into the actual DOM.
Doing that is just a plain type error.

The cases where I think libraries like dom.js make sense is:
- in an environment that doesn't have a real DOM at all (e.g. NodeJS)
- in a sandboxed environment (like SES) where the sandboxed code cannot
ever access the real DOM.

In neither of these cases, the problem comes up.

In general, when you're emulating a system, it is almost never the case
that emulated objects must run on the *real* system directly. The real
system doesn't know about the emulation's implementation layer, so it can't
"interpret" the emulated object.

Choosing to automatically unwrap the proxy and pass in the wrapped DOM node
is dangerous. As Brendan mentioned, it breaks the proxy's abstraction. In
that regard, I agree with Jason that it's best to think of a proxy's
"target" as an encapsulated, internal piece of proxy state that should
never leak. At best, external code can use the target to derive some simple
information (like typeof does on direct proxies).

The suggestion of allowing proxies to hook into the deeper DOM protocols
and intercept inner calls (via symbols or however else you want to do that)
seems to me a terrible idea.
It seems at least some people here agree.

To summarize, to me, emulating the DOM is as much about proxying DOM
objects as it is about wrapping the actual DOM functions (like
appendChild). It's the job of the wrapped functions to recognize their
proxies, unwrap them, and pass their unwrapped counterpart into the real
DOM.

Cheers,
Tom
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