[[Invoke]] vs [[Get]]

Till Schneidereit tschneidereit at gmail.com
Sat Jun 8 03:30:11 PDT 2013

At the last TC39 meeting, an agreement was reached for proxies to support
an Invoke trap. I'm currently implementing this trap in SpiderMonkey[1] and
realized that there's one conceptual issue that has to be decided in the

Currently, the [[Get]] trap naturally handles method calls on the proxies,
as these are gets followed by calls to the returned function value. With
proxies also being able to trap [[Invoke]], things become less clear, with
four possible options:

1. for method calls, only the [[Invoke]] trap is called
2. if an [[Invoke]] handler exists, it is called, otherwise a [[Get]]
handler is called, if that exists
3. like 2., but additionally, if the [[Invoke]] handler doesn't return a
callable value, the [[Get]] handler is called
4. like 3., but with reserved order of handler invocation: if a [[Get]]
handler exists, it is called. If the result isn't callable, or [[Get]]
isn't handled, an [[Invoke]] handler is called, if one exists

The first of these options seems conceptually cleanest to me, but I do
think that the other options (perhaps except for the last one) can be
argued for to some extend, too: the abstract operation `Invoke`, via its
step 5, `GetMethod`, contains a call to the [[Get]] operation, after all.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=878605
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