Yes!!
On 6/25/14, 8:15 AM, Jason Orendorff wrote:
We're considering building a JavaScript API for dynamic analysis of JS
code.
Here's the sort of thing you could do with it:
- Gather code coverage information (useful for testing/release mgmt?)
Yes! We'd absolutely love to show code coverage in the debugger's source
editor! We've played with implementations based on
Debugger.prototype.onEnterFrame and hidden breakpoints, but it is pretty
slow and feels like a huge hack. Would this work likely yield improved
performance over that approach?
- Trace all object mutation and method calls (useful for devtools?)
Absolutely! A thousand times yes! We've made a prototype tracing
debugger (again, based on Debugger.prototype.onEnterFrame) but (again)
it is pretty slow so it isn't pref'd on by default (a few other reasons
as well; it needs polish).
We (devtools) have talked a lot about wanting to be the "best printf
debugger" to lower the barriers to entry for people who normally print
debug. Manually adding printfs is like a really crappy, temporary,
tracing debugger; I see huge opportunities for developer productivity here!
(Flip devtools.debugger.tracer to see the prototype:
http://i.imgur.com/E3jtaxv.jpg)
- Record/replay of JS execution (useful for devtools?)
I'm drooling aaaahhhhhhhhhh
- Detect when a mathematical operation returns NaN (useful for game
developers?)
This would be a great fit for a console warning, IMO.
Note that the API would not directly offer all these features.
Instead, it
would offer some powerful but mind-boggling way of instrumenting all JS
code. It would be up to you, the user, to configure the
instrumentation, get
useful data out of it, and display or analyze it. There would be some
overhead
when you turn this on; we don't know how much yet.
Would the API be something like DTrace? Just want to figure out what
kind of thing we are talking about here.
We would present a detailed example of how to use the proposed API,
but we are
so early in the process that we're not even sure what it would look like.
There are several possibilities.
We need to know how to prioritize this work. We need to know what kind
of API
we should build. So we're looking for early adopters. If that's you,
please
speak up and tell us how you'd like to instrument JS code.
/me raises hand
Mostly interested in tracing calls and mutations. Also code coverage,
but to a bit of a lesser extent.
Record/replay is such a holy grail (eclipsed only by "time traveling" /
reverse and replay interactive (as opposed to a static recording)
debugging with live, on-stack code editing) that I hesitate to even get
my hopes up...
Nick
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