Unexpected token u in JSON

[MuTsunTsai]

Describe the bug

In my project I'm running unit tests with mocha + ts-node + @swc/core. Not sure if this is a bug of @swc/core or of ts-node, but ever since I updated to @swc/core 1.3.50+ it shows the following error when I try to run the test:

Input code

No response

Config

No response

Playground link

No response

Expected behavior

There shouldn't be an error.

Actual behavior

No response

Version

1.3.50,1.3.51,1.3.52

Additional context

Although the source of the error seems to be in ts-node, but downgrading to @swc/core 1.3.49 solves the issue.

[kdy1]

Please provide a repro

[MuTsunTsai]

@kdy1 Well, if I must, the repo is https://github.com/bp-studio/box-pleating-studio in DEV branch. To reproduce the error:

1. Use PNPM to perform pnpm install after cloning the repo and checking-out DEV branch.
2. Run pnpm test. Mocha should run without problem.
3. Run pnpm update -L to force update @swc/core to the latest version.
4. Run pnpm test to see the error.

Please reopen the issue if that's not too much to ask for.

[kdy1]

Thank you, but as it's not a minimal repro, I don't think this can be fixed shortly.

https://antfu.me/posts/why-reproductions-are-required

[MuTsunTsai]

Maybe this will help. I've traced the issue down to this part:

swc/node-swc/src/index.ts

Lines 232 to 236 in d5c0dec

	return bindings.transformSync(
	isModule ? JSON.stringify(src) : src,
	isModule,
	toBuffer(newOptions),
	)

I believe that this transformSync method is in Rust so that the debugger won't allow me to step in further. In version 1.3.49, this always returns an object with both code and map properties, where the map could be an empty map like this:

{"version":3,"sources":[],"names":[],"mappings":""}

But since version 1.3.50, it could return an object with only code but not map (i.e. the property is missing), if the file being processed essentially contains no executable code (for example, it contains only TypeScript definitions), and it is because of the missing map that results in later error in ts-node. This breaking change of behavior feels more like unplanned to me.

[MuTsunTsai]

@kdy1 Following the idea above, I've created a minimal repo here:
https://github.com/MuTsunTsai/swc-bug-demo

After npm install, run npm run test to see the error. Downgrade to v1.3.49 and there's no error.

[kdy1]

Thank you so much for the investigation! This is an easy issue and I'll fix this tomorrow

[kdy1]

Investigation takes an unknown amount of time, while fixing is very easy in most cases.

[MuTsunTsai]

@kdy1 True, and glad that it is also the case here. I admit that it did take me a while to go this far. Hope to see it fixed in the next version.

[MuTsunTsai]

Confirm fixed in v1.3.53

[swc-bot]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.