934223 - Implement revocable proxies

Status: RESOLVED DUPLICATE of bug 978279

(Core :: JavaScript Engine, defect)

Description

[David Bruant]

Initial spec http://wiki.ecmascript.org/doku.php?id=strawman:revokable_proxies (but look at the latest ES6 draft for the most up-to-date version if changes happen).

In a nutshell:
  let { proxy, revoke } = Proxy.revocable(target, handler);
  proxy.foo // traps
  revoke()  // always returns undefined
  proxy.foo // throws TypeError: "proxy is revoked"

Comment 1

[Bobby Holley (:bholley)]

Is there anything this lets you do that isn't already possible with Direct Proxies? Or is this just streamlining the revocable proxy pattern with a native API?

Comment 2

[David Bruant]

(In reply to Bobby Holley (:bholley) from comment #1)
> Is there anything this lets you do that isn't already possible with Direct
> Proxies?
It enables GC'ing of the target. For the longer explanation, see [1], but in a nutshell, whatever handler semantics is implemented, the proxy always keeps a reference to its target. Revoking the proxy allows to break this reference.
Some handler semantics could mean the same thing, but it'd be hard (undecidable? impossible?) for the JS engine to understand the handler enough to know that the target is collectable even when the proxy is still around.

> Or is this just streamlining the revocable proxy pattern with a native API?
That's just a nice side-effect :-)


[1] https://mail.mozilla.org/pipermail/es-discuss/2012-August/024344.html

Comment 3

[Bobby Holley (:bholley)]

Makes sense. This is trivial for us to implement. We should probably just use DeadObjectProxy and updates its throwing semantics to match whatever the ES spec wants.