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petegrif
Very interesting. The reason we didn't want to use GWT was precisely because of the difficulty of debugging. This is a big step in the right direction.

The talk emphasizes debugging js as output of another language. But did I understand correctly that it can be used to debug js programs directly? Makes sense and would be awesome.

Great job - again.

cromwellian
Yes, if you have regular hand-written JS, and let's say you run UglifyJS, YUI Minifier, or Closure Compiler, to generate compact, optimized JS, with source maps, the debugger will show the original JS source.

The demo I showed also is using SuperDevMode for GWT, which dramatically speeds up the compile. Notice, I compile a project (the Mail sample) of several hundred Java source files in less than 2 seconds.

petegrif
Thanks Ray. Nice tool for reverse engineering then. :)
duotor
This is not reverse engineering. The compiler generates the compact javascript along the SourceMap.
YmMot
I think what they mean is say you have a bit of obsfucated, minified, and or complex JavaScript code you want to reverse engineer. You can start writing a an "unobsfucated" version in JS, pseudocode, whatever you wanted....and use source-mapping to switch between looking/working on that and the original in the debugger all while the browser is actually executing the original code (negating worries that your translation is incomplete or behaves differently, these are concerns during certain steps of the process...obviously you want to end up with a perfect translation in the end)

That would be a useful tool in reverse engineering.

lapusta
Will we see SuperDevMode in GWT 2.5?
asadotzler
This is coming to Firefox as well. I remember watching an intern present some early prototypes a year or so ago.
cromwellian
Do you know if Firefox is making progress, or is it stalled?
masklinn
https://wiki.mozilla.org/DevTools/Features/SourceMap

It's reached a demoable state, but is not complete and can't be landed: it was worked on by an intern and the intern has now gone back to life.

cromwellian
:( That happens at Google often too. We are always happy when Summer-of-Code rolls around and we get interns again. :)
andybak
That old 'unpaid labor' thing working out well for you then!
pufuwozu
Successful Google Summer of Code students get paid for their projects.
TruthElixirX
That old 'unpaid labor' thing working out well for you then!
ilaksh
Do SourceMaps allow you to debug CoffeeScript in Chrome and if so what exactly do you do?
YmMot
It allows you to debug any language that compiles to JavaScript, with the caveat that the language's compiler must output source-map compatible code (or the source must be run through some other program that somehow does it).

It's a pretty simple but extremely useful idea. To use your example, the CoffeeScript compiler knows exactly which bits of generated JavaScript correspond to exactly what bits of CoffeeScript code, so the compiler can output a mapping that says "these lines of JavaScript are the result of this line of CoffeeScript...so when you step through this CoffeeScript line, execute these X lines of JavaScript, if an error occurs in these X lines of JavaScript show there error as occurring on line Y of the CoffeeScript code.

I haven't done much front-end lately so I'm not sure what if any the CoffeeScript compiler has in terms of generating source maps, but if browsers are supporting it I imagine it won't be long if it's not here already.

cromwellian
AFAIK, the CoffeeScript guys are working intensely on it. To some extent, any language which has accurate source position information in its parser/AST, and uses Closure Compiler on the backend can get source map output almost for free.
YmMot
Yea I would imagine they are. I haven't been following the development like I used to, so thanks.

How accurate does the source position information have to be? I don't think the CoffeeScript lexer can output column numbers (at least it didn't last time I checked) and the re-writing stage might give them a little trouble. Should be doable though.

Guess I should head over to github and see how they're doing and start reading the SourceMap spec.

cromwellian
IIRC, you only need line number and filename. The map is JS <startline/startcol> -> Source <line,filename>
jschorr
However, you can embed extra metadata for your specific source language if you wish, via the extension mechanism. Debug tools can then read the metadata in to do extra neat kinds of mapping (such as collapsing "native" stack frames).
thomasfl
I would love to debug coffescript with this.
duotor
Debugging GWT through the dev mode used to be slow and painful. The SourceMap will definitely make GWT development more cool.

Great move guys!

nailer
Isn't source mapping part of ES6?
pufuwozu
SourceMap is a tool feature - no need to modify a language (except if you count making the compiler output SourceMaps).

Maybe you're thinking about how the Traceur ES6 to JavaScript compiler outputs SourceMap information?

grayrest
Is this in Canary? I know you said "soon" but I'm always unsure if soon means soon in stable/dev or soon in canary.
pufuwozu
In Canary 19.0.1074.0 for me
zem
hope js_of_ocaml implements this.
cromwellian
BTW, thanks for all the positive feedback. If you are working on a language translator to JS and end up including sourcemap support, just ping me on G+ and I'll put it into a directory of source-map enabled tools.