Inaccurate sourcemaps for sources under node_modules

[hbenl]

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #85~20.04.1-Ubuntu SMP Mon Jul 17 09:42:39 UTC 2023
Binaries:
  Node: 18.16.0
  npm: 9.5.1
  Yarn: 1.22.18
  pnpm: 8.6.7
Relevant Packages:
  next: 13.4.12
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: N/A
Next.js Config:
  output: N/A

Which area(s) of Next.js are affected? (leave empty if unsure)

No response

Link to the code that reproduces this issue or a replay of the bug

https://github.com/replayio/nextjs-inaccurate-sourcemaps

To Reproduce

• npm run build
• upload .next/static/chunks/main-26f0defd1fe4aedb.js[.map] to https://evanw.github.io/source-map-visualization/
• select webpack://_N_E/node_modules/@swc/helpers/esm/_interop_require_wildcard.js as the original source
• for comparison, upload .next/static/chunks/pages/_app-27374a655d3a2ebe.js[.map] to https://evanw.github.io/source-map-visualization/ and select webpack://_N_E/src/_interop_require_wildcard.js

Alternatively, you can look at these sourcemaps here:

• webpack://_N_E/node_modules/@swc/helpers/esm/_interop_require_wildcard.js: https://app.replay.io/recording/5072126e-507f-43d9-8c9e-889ad2bfd0dd/sourcemap/o4-80-c194bb-ec5646
• webpack://_N_E/src/_interop_require_wildcard.js: https://app.replay.io/recording/5072126e-507f-43d9-8c9e-889ad2bfd0dd/sourcemap/o6-6-c194bb-5d30d7

Describe the Bug

I've noticed that the sourcemaps produced by Next.js are inaccurate for some sources, making it impossible for debuggers to show scopes with original (rather than generated) variable names.
This seems to be the case for all sources under node_modules.
To reproduce the problem I've used create-next-app to create an app and copied node_modules/@swc/helpers/esm/_interop_require_wildcard.js to src/_interop_require_wildcard.js. This is what a sourcemap visualizer shows for these two copies of the same source:

node_modules/@swc/helpers/esm/_interop_require_wildcard.js:

src/_interop_require_wildcard.js:

Expected Behavior

Sourcemaps for sources under node_modules should be accurate.

Which browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

No response

[leerob]

Have you determined what version this started to be an issue for you?

[jridgewell]

This seems like it's defaulting to a low fidelity source map for inputs which don't have a source map. It affects node_modules/@swc/helpers/…, but not next/dist/compiled/… (which has an source map to the TS sources). I think it's likely that we're not transforming the source (because it's inside node_modules), so we don't generate a high-fidelity AST mapping.

[hbenl]

Have you determined what version this started to be an issue for you?

No, I don't know if we ever got better sourcemaps - we rarely debug sources under node_modules (because it's mostly third-party code) but recently we had a few issues where we wanted to understand the behavior of some code under node_modules and that's when we noticed the issue.

I also wanted to highlight something peculiar: while in the screenshots above it looks like the inaccurate sourcemap is (mostly) using one mapping per line, it actually uses the same number of mappings per line as the accurate sourcemap - but the mappings all point to column 0 in the original source (have a look here and here).
Initially I thought that maybe Next.js intentionally creates "low fidelity sourcemaps" for code under node_modules to reduce the overall size of the sourcemaps but since those sourcemaps have the same number of mappings the size won't be reduced (at least not significantly).

[bvaughn]

but since those sourcemaps have the same number of mappings the size won't be reduced (at least not significantly).

I wonder if it's less a concern about size and more a concern about build times?

[markerikson]

I'm seeing pretty similar issues myself (ironic given that I work with Holger and Brian!).

Per markerikson/react-prod-sourcemaps#4 (comment) , I have created a package that contains backported sourcemaps for existing versions of ReactDOM.

I can view one of my sourcemaps by itself, and see that each individual function argument is mapped correctly.

However, when I added the build plugin to swap the ReactDOM sourcemap to our own Next + Webpack config, the results show that the entire function declaration line is getting mapped as a single block, rather than separate mappings for the function name and each separate argument.

[markerikson]

React just merged my PR to start generating sourcemaps:

• Generate sourcemaps for production build artifacts facebook/react#26446

and I see that Next has updated to pull in that build of React:

• Update React from 2983249dd to 7508dcd5c #58216

But, as long as this issue exists, the React sourcemaps in Next will likely be semi-broken because function arguments are losing their mappings.

[jridgewell]

If React is now publishing the source maps as part of the NPM module, then all we would need to do is hook it up so that input code could reference source maps, and feed that through to the output assets. I think the only line that needs to be changed is https://github.com/vercel/turbo/blob/fffcfd86baf9f150f7d443e96f4d18733311352f/crates/turbopack-ecmascript/src/lib.rs#L693. We'll need to check if there's a sourceMappingURL in the referenced source file and link the source map, but that should be easy.

[hbenl]

@jridgewell If I understand your comment correctly, this would include the sourcemaps for React but it wouldn't change the fact that those sourcemaps will be semi-broken (which is what this issue is about).

[jridgewell]

I tried reproducing this with latest 14.0.2, and I can't find a broken _interop_require_wildcard in the maps. At some point, we started generating full maps for all inputs (regardless of if they're in node_modules). We still don't use an inputs own map.

[hbenl]

I also tried with 14.0.2 - here is the branch and here is a recording and this link shows that _interop_require_wildcard.js still has an inaccurate sourcemap.

[jridgewell]

Oh, this is using webpack? I thought this entire thread was about Turbopack. 😅

[bvaughn]

This issue continues to be a pain point for us, @leerob. Any chance someone at Vercel could take a look? If you need any more info from us, we'll be happy to provide it.