From:  Brian Slesinsky <>
Date:  13 Mar 2014 08:30:36 Hong Kong Time

Re: Source maps in ES7?


On Wed, Mar 12, 2014 at 5:04 PM, Fitzgerald, Nick

> On 3/12/14 2:40 PM, Brian Slesinsky wrote:
> > Interesting. I think the sticky part will be providing the
> > getLocals(), getDisplayValue(), and eval() functions. I think it's
> > right that they should be language-specific JavaScript code provided
> > by the compiler, since this allows a lot of flexibility. But which
> > JavaScript environment should they run in and what do they have access
> > to? I think it would have to be a sandbox of some sort and there may
> > be security implications for curious developers who run untrusted code
> > with the debugger window open.
> I was imagining it would run in the frame the debugger was paused in.
> That way it would have access to whatever JavaScript locals exist.

However that means the debugger's view of things could possibly be broken
by someone monkey-patching a prototype in the web page itself (or possibly
by an extension changing something). A debugger or a debugger plugin should
work even when things have gone very wrong in the program being debugged.
But it's hard to write code sufficiently paranoid to work in any

> Perhaps I am missing something, but if you are already running a site's
> code, why would they need to send the bad stuff via the debugging
> information?

We would be creating a place to hide JavaScript code where nearly all web
developers wouldn't suspect it, which sounds like an interesting target for
a hacker.

It would be a problem if it somehow ran with higher privileges than the
code being debugged. For example, suppose the debugger called the plugin
code when it's looking at the local variables of a function that's running
in an iframe that came from a different origin? Now we have an interesting
way to get around same-origin restrictions.

So I think running each compiler's custom code as a separate low-privilege
extension makes the most sense, both to protect it from outside
interference and to make sure it can't do anything malicious.

- Brian