DevTools/Features/SourceMap

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Feature Status ETA Owner
Source Map planning 2011-06-01 Kevin Dangoor

Summary

Nearly every web developer starts with JavaScript in one form during development and then produces JavaScript in another form for production use. Most often, this is a case of joining files together and minifying the output to reduce the number of requests the browser needs to make and how much data the browser needs to transfer. Another example of JavaScript starting in one form and ending up in the final form used by the browser is in PHP scripts which will sometimes generate bits of JavaScript as they generate their pages.

When JavaScript gets an error, being able to help the web developer out by showing them where the error occurred in the original source file (PHP script or unminified, still-separate JavaScript for example) would save the developer time every single time they hit an error. The Web Console log messages also include links to the line that generated the log message, so these messages could link back to the original source lines as well.

For a long time, there have been languages that have compiled to JavaScript, and recently there has been a surge in support for these languages led by CoffeeScript. CoffeeScript may seem like a fringe language today, but it will soon be shipping with Ruby on Rails 3.1, giving thousands of programmers a reason for developing with something other than JS.

When developing an application in CoffeeScript, any time an error is hit the browser tells the user the location of the problem in the JavaScript file, not the source file that the user is working with. One new language even went so far as to ensure that the lines numbers in the generated JS match up with the lines numbers in the original source file, which is an unfortunate thing to optimize for.

The solution to this problem is a mapping from the generated JS to the original source files.

Team

  • Feature Manager: Kevin Dangoor (irc: kdangoor)
  • Lead Developer: Nick Fitzgerald (irc: fitzgen)
  • Web Console Contact: David Dahl (irc: ddahl)
  • SpiderMonkey Contact: Jason Orendorff (irc: jorendorff)

Release Requirements

  • SpiderMonkey needs to output column in addition to source/line.
  • When a sourcemap is available, error messages in console will link to the original source, not the generated JS
  • Likewise, console log output will link back to the original source
  • Patches for CoffeeScript and UglifyJS will be used to produce examples
  • A specification for generated scripts to refer to their mappings and a mapping format

Link title

Next Steps & Open Issues

A security review should be conducted before we have too much technical debt.

Now that there is an evolving spec for the source map file format, the first step is to write the core libraries to generate and consume/query source maps. These core libraries should be very general and reusable, so the plan right is to use the Common JS AMD format.

Next, a patch should be written for the web console to support mapping logged messages and uncaught errors back to their original source. It is still undecided, from an API/architecure point of view, who will check whether a given script has an associated source map, and how they will notify the potential consumer of the source map that it exists or mark the script as having an associated source map. This issue will rise to the surface at this stage.

As time progresses, and dependencies are met (or not), the order of the following still needs to be decided:

  • Integration with the debugger.
  • Integration with UglifyJS.
  • Integration with CoffeeScript.

Related Bugs & Dependencies

TBD (status page link)

  • Bug 626932: Shrink JSParseNode - Would add character-offset to SpiderMonkey (as opposed to line/col).
    • Has an incomplete patch by njn. Doesn't compile.
  • David Flanagan has been working on adding "//@line ###" style comment support to SpiderMonkey which is being tracked by this bug.

Risks

  • We could end up with two competing formats if other browsers pursue this at the same time and we're not aware of it.

Designs

The source map file format spec originally proposed by Google. I (fitzgen) have edit permissions and am working with them on making something we can all agree upon.

This feature is related to the Script Origin Tracking proposal, because the mapping should be able to accommodate the many ways in which JavaScript can come into existence on a page.

Test Plans

TBD

Non-goals

  • Ideally, the source mapping file will also work for mapping LESS and similar to CSS. However, implementing the user interface for style source mapping is out of scope for this particular feature.

Other Stuff

  • Brendan and Jim Blandy have suggested that using a separate mapping file will work better than including the mapping in the generated source.
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