Relative paths become absolute paths when using a source map with empty string as the sourceRoot with SourceMapConsumer

[Arnavion]

Edit: Fixed title to reflect the underlying issue (empty string as sourceRoot).

The issue is that loading a source map with sourceRoot: "" into a SourceMapConsumer and then querying it via originalPositionFor() returns a source that is an absolute path. For example, if the path in the source map was "foo.ts", then consumer.originalPositionFor(...).source is "/foo.ts"

So there are two things here:

• Empty string sourceRoot is no longer interpreted the same as it used to be.
• Empty string sourceRoot causes relative paths to become absolute paths.

Original issue text below:

---

I start with a sourcemap generated from Typescript that has:

"sources":["utility.ts","web-worker.ts","parts.ts","parser.ts","renderers-clocks.ts","renderers-null.ts","renderers-web.ts","renderers-default.ts","types.ts","libjass.ts"]

I then run the Javascript and this source map through UglifyJS for some transforms. When writing out the source map from UJS, I get this:

"sources":["/utility.ts","/web-worker.ts","/parts.ts","/parser.ts","/renderers-clocks.ts","/renderers-null.ts","/renderers-web.ts","/renderers-default.ts","/types.ts"]

Note the relative paths (i.e., relative to current directory) have become absolute paths. This breaks loading the original JS in browsers because they try to fetch the original source using the absolute path instead of relative to the sourcemap URL.

git bisect reveals this was caused by 0235bb0

[ai]

Yeap, same problem breaks PostCSS test and can affect on Autoprefixer users. Should I set sourceRoot or it is just bug and will be fixed?

[ai]

/cc @lydell

[Arnavion]

I found out it's because the sourcemap output from Typescript sets sourceRoot to "". If I first set it to null or remove that property entirely and then run it through UJS, the generated sourcemap is fine.

I do override sourceRoot to null when passing the TS-generated sourcemap to UJS:

var output = {
    source_map: UglifyJS.SourceMap({
        file: path.basename(sourceMapFile.path),
        orig: sourceMapFile.contents.toString(),
        root: null   // <--
    }),

which uses that parameter to create a SourceMapGenerator as so:

var generator = new MOZ_SourceMap.SourceMapGenerator({
    file       : options.file,
    sourceRoot : options.root   // <--
});

so I'm checking why this override is not taking effect.

Edit: It's because UJS only uses the sourceRoot overriding parameter for the SourceMapGenerator (i.e., the output source map), not the SourceMapConsumer (i.e., the input source map). The Consumer gets its sourceRoot from the sourcemap string that's passed in, so it has an empty string as the sourceRoot. That makes sense.

[Arnavion]

So the reason is that TypeScript outputs source maps with the sourceRoot set to "" by default. If this is a deliberate change in 0.1.35 to not treat empty string sourceRoot as the same as null, then I can file a bug with TypeScript to default to outputting null.

[ai]

But I didn’t set sourceRoot in PostCSS, but have same absolute URL.

[lydell]

The case where sourceRoot is an empty string is a bug in source-map. Specifically, in its util.join(aRoot, aPath) function. If aRoot is an empty string then currently "/" + aPath happens to be returned. It should work as Node.js path.join() and url.resolve() where the empty string is treated as ".". I’ll fix this at once.

Still, I think it is pretty in-elegant of TypeScript to set the sourceRoot property to the empty string when they just could have omitted it. It’s always fascinating to see how much real code relies on edge cases :)

@ai I’ll fix the above issue first, then I’ll look into why your PostCSS tests fail.

[Arnavion]

Cool. Thanks for confirming.

[lydell]

@ai I pulled #124 and #121 and then ran the postcss test suite without errors :)

[lydell]

@Arnavion It would be really nice if you could pull #124 and test that it really solves the problem for you.