SourceMaps in Chrome allow debug of any source language compiled to JS | Hacker News Reader

SourceMaps in Chrome allow debug of any source language compiled to JS

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180 pointscromwellian32 comments

A small demo of the SourceMaps support in Chrome Dev Tools which allows languages which are cross compiled to Javascript (Java, CoffeeScript, Objective-J, ClotureScript, even minified-JS) to be source level debugged in the browser.

Firefox will also be sporting support for SourceMaps.

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If you'd like to help with the Firefox implementation, there's code and specs here https://wiki.mozilla.org/DevTools/Features/SourceMap

This is coming to Firefox as well. I remember watching an intern present some early prototypes a year or so ago.

Do you know if Firefox is making progress, or is it stalled?

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.

:( That happens at Google often too. We are always happy when Summer-of-Code rolls around and we get interns again. :)

That old 'unpaid labor' thing working out well for you then!

Successful Google Summer of Code students get paid for their projects.

That old 'unpaid labor' thing working out well for you then!

Demo of another SourceMap effort used to debug Coffeescript: http://www.youtube.com/watch?v=UAGAB-yT0lQ

Do SourceMaps allow you to debug CoffeeScript in Chrome and if so what exactly do you do?

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.

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.

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.

IIRC, you only need line number and filename. The map is JS <startline/startcol> -> Source <line,filename>

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).

Here's a link to the CoffeeScript discussion on source maps (with a working alternate solution).

https://github.com/jashkenas/coffee-script/issues/558

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.

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.

Thanks Ray. Nice tool for reverse engineering then. :)

This is not reverse engineering. The compiler generates the compact javascript along the SourceMap.

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.

Will we see SuperDevMode in GWT 2.5?

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.

In Canary 19.0.1074.0 for me

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.

hope js_of_ocaml implements this.

Link to documentation? If I wanted to build my own little JavaScript preprocessor and make it use SourceMaps in Chrome, how would I know the file format(s)?

Oh hey -- here's Google's current draft, linked from the FireFox bugs about it: https://docs.google.com/a/ooyala.com/document/d/1U1RGAehQwRy...

Debugging GWT through the dev mode used to be slow and painful. The SourceMap will definitely make GWT development more cool.

Great move guys!

I would love to debug coffescript with this.

Isn't source mapping part of ES6?

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?

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