List source map implementations

[littledan]

If we're trying to harmonize source map implementations, it would be useful to list all of the implementations we can find, so that we can then compare them. This includes both generators and consumers.

[bomsy]

These would also be super helpful for understanding the systems to be tested.

Here is an implementation from mozilla used in firefox
https://github.com/mozilla/source-map

[sjrd]

I'll put forward the Scala.js compiler as generator.

Some of our interpretations are:

• Every source tree carries its source position through transformations (counting the entire pipeline, there are dozens of transformation phases) and eventually reaches every JavaScript AST node.
• When printing a JS AST node, we emit source map segments that point to its position for all the text it generates, unless overridden by children nodes. So for example, an expression (a + b) attaches the parens, spaces and + sign to the position of the AST node for +, while a and b are attached to the positions of the identifier AST nodes.
• We only give original names to the definition of identifiers. In let a = 1; a only the first a receives an original name in the source map.
• Our source files are UTF-8 encoded usually, and strings in Scala are ~UTF-16 (like JS). In terms of output, we always emit ASCII.

In terms of implementation, perhaps one key aspect is that it is specialized for an incremental compilation pipeline. When changing part of a source program, the entire Scala compilation pipeline is incremental end-to-end, and that until re-pretting JS AST nodes and generating source maps. Source maps are actually a pain point here, since they must be regenerated from top to bottom every time. We have an intermediate data structure consisting of Fragments to make that as efficient as we could.

You can see our implementation here:
https://github.com/scala-js/scala-js/blob/v1.13.1/linker/shared/src/main/scala/org/scalajs/linker/backend/javascript/SourceMapWriter.scala
If you're not familiar with Scala, it's worth noting that this file looks nothing like typical Scala code. This is almost C code written with Scala syntax, because of how performance-sensitive it is.

[7rulnik]

There is also a fork of source-map — https://github.com/7rulnik/source-map-js

[mitsuhiko]

This is our Rust library: https://crates.io/crates/sourcemap

The most notable users of this library to the best of my knowledge are Sentry, SWC and Deno.

It's pretty low level, we have higher level wrappers on top of that (js-source-scopes, symbolic, and symbolicator) for consumption.

[jaro-sevcik]

Here is Chrome DevTools implementation: https://source.chromium.org/chromium/chromium/src/+/main:third_party/devtools-frontend/src/front_end/core/sdk/SourceMap.ts

[connor4312]

There is also https://www.npmjs.com/package/@jridgewell/source-map which is namely used in Terser. It tries to be compatible with Mozilla's source map library, but I have found some variance in the past.

[jkrems]

Will there be interesting differences between uses of the libraries when it comes to what we'd want to specify? E.g. will all users of mozilla/source-map generate "spec compliant source maps in all ways we care about" as soon as mozilla/source-map has been updated in accordance with a future hardened spec?

If not, it may also be helpful to collect things like postcss which indirectly produce source maps.

[jridgewell]

I maintain:

• @jridgewell/trace-mapping (40m downloads/week)
• @jridgewell/gen-mapping (40m downloads/week)
• @ampproject/remapping (24m downloads/week)

[ochameau]

While trying to revive mozilla original source-map implementation, I went throught all major source map implementations I could easily find online and come up with this summary:
https://docs.google.com/document/d/1cRNI5s9ECI6V7argqw0g7TDp79gNrOZiVYLIRDD9FlA/edit#heading=h.3dsnp8frk1tz
You will see links to sources of each of them and some recap about the specifics of each.

[sokra]

https://github.com/webpack/webpack-sources has a SourceMap implementation here: https://github.com/webpack/webpack-sources/blob/main/lib/SourceMapSource.js

[legendecas]

SourceMap in Node.js: https://github.com/nodejs/node/tree/main/lib/internal/source_map

[mischnic]

https://github.com/parcel-bundler/source-map is used by Parcel and Lightning CSS