To clarify, this is about XPConnect JS modules (loaded by Cu.import[1]) and
components, not ES6 modules[2], which already work this way.
[1]:
https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/Using
[2]: https://hacks.mozilla.org/2015/08/es6-in-depth-modules/
On Thu, Aug 31, 2017 at 11:14:34AM -0700, Kris Maglione wrote:
A quick heads-up on some incoming changes to the JS component loader
that may affect some existing JSM or JS component code:
1) As of bug 1394556[1], all JS module and component scripts are
automatically compiled in strict mode[2]. While I fixed all of the
resulting issues that showed up in automated tests, new issues may
turn up in code with incomplete test coverage. Please be on the
lookout for errors like "TypeError: this is null", "TypeError: setting
a property that has only a getter", and "ReferenceError: assignment to
undeclared variable", and if you see any, file bugs blocking bug
1394556.
2) Andrew McCreight has been working on changing the component loader
to load most modules and components into a single global[3], with each
module still getting its own top-level variable scope (similar to a
with() block). Most of the blockers for this have been resolved, and
it should hopefully be ready to land within the next week.
This change will significantly reduce the memory overhead of JS
modules, along with the performance overhead of creating dozens module
globals at startup, and of creating thousands of cross-compartment
object wrappers at runtime.
But it does so at the cost of decreased isolation among module
scripts. All (non-add-on) modules will now share prototypes for
built-in objects. It will be possible to define objects on the shared
global that affect all modules. getGlobalForObject will no longer
return the root scope for a module. In short, it's possible we will
see subtle issues as a result of this change (as we often saw on b2g,
which its module loader differed from the desktop module loader), so
please be on the look-out over the next few weeks.
Thanks.
-Kris
[1]: https://bugzil.la/1394556
[2]:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode
[3]: https://bugzil.la/1186409
--
Kris Maglione
Senior Firefox Add-ons Engineer
Mozilla Corporation
For every prohibition you create you also create an underground.
--Jello Biafra
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