undefined is passed for the "context.metadata" field in a decorator metadata

[arklanq]

Describe the bug

When switching from tsc to swc to transpile TypeScript to JavaScript code I have noticed that context.metadata argument for decorators becomes undefined.

More context:

• I am trying to replace tsc with swc for faster source code transpilation in a relatively modern Node.js app.
• I am using '2022-03' decorators ("stage 3 decorators"), see swc config below.
• I am using the latest version of TypeScript (5.2+).
• I am using SWC within Rollup bundler, thanks to rollup-plugin-swc3 package.
• I have polyfilled decorator-metadata feature for my node.js with core-js/proposals/decorator-metadata-v2.js.

Input code

type ClassTarget = abstract new (...args: unknown[]) => unknown;

function inject<Class extends ClassTarget>(
  target: Class,
  context: ClassDecoratorContext<Class>
): void {
  console.debug(context.metadata); // undefined
}

@inject
class App {

  foo() {
    return 'bar';
  }

}

const app = new App()
app.foo();

Config

const config = {
    // --- Plugin options
    include: /\.ts$/,
    exclude: /node_modules/,
    tsconfig: 'tsconfig.json',
    // --- SWC options
    env: {
      targets: 'node >= 18.14',
      // 'entry' mode imports too much polyfills
      // 'usage' mode doesn't work as expected
      mode: undefined,
      coreJs: coreJsPackageManifest.version,
      dynamicImport: true,
    },
    jsc: {
      parser: {
        syntax: 'typescript',
        tsx: false,
        decorators: true,
        dynamicImport: true,
      },
      transform: {
        // Stage 3 decorators
        decoratorVersion: '2022-03',
      },
      externalHelpers: true,
      keepClassNames: true,
      // Use `env` option instead
      target: null,
    },
    minify: PRODUCTION_BUILD,
    sourceMaps: true,
  }

Playground link

https://play.swc.rs/?version=1.3.74&code=H4sIAAAAAAAAA02QwU7EIBCG7zzF3BYSw95b22jWR%2FBm9jCF6aa6Dg1MdzXGd3eENTHhwPzwffxBPleCwxlLecZ8IoEBcCqSMQgwXcF67%2FWgdLDxG6crvxwdDOPf1BszbxxkSQwLv1KQ%2ByoD%2BhDiWP6rR2sApG67lt9pEBKLXr4lTxRSRkn50OJmG43r4JKWCF%2BNKOlMPtK0neyN9%2B8kGFHQ9bDfa71I88IUzbcxD62ZCbXZ47qqRj1zStZVI0Am2TLDbsK86zVRStfvSwKowFA%2FQ1HrjM6%2Bsr35AYrFObg%2BAQAA&config=H4sIAAAAAAAAA22QPQ%2BCQAyGd3%2FFpbMaQQdjoouLLo7uDRSDwt2lrQZC%2BO8efiAap7bv0%2B9mZAycJYGVaYIbAo8sxH0cFKmtYhUU0NqTJJx7hfGbqnQow0Ko11JKHKM6loCUrwNSWyzzZF96x%2FoF26cBZbSSOS6HK%2FQNj8SSO9stE8%2FieDKbw3c5VUpssdhR4elnPlyI%2FLZAkQOWNGSPaiB7%2B%2FxBkU%2BkXRJYl5LZrE20nEaL17y%2Fl3SdRu0dcYStD1QBAAA%3D

SWC Info output

Operating System:
    Platform: darwin
    Arch: arm64
    Machine Type: arm64
    Version: Darwin Kernel Version 22.6.0: Wed Jul  5 22:22:52 PDT 2023; root:xnu-8796.141.3~6/RELEASE_ARM64_T8103
    CPU: (8 cores)
        Models: Apple M1

Binaries:
    Node: 18.17.0
    npm: 9.8.1
    Yarn: 1.22.19
    pnpm: N/A

Relevant Packages:
    @swc/core: 1.3.82
    @swc/helpers: 0.5.1
    @swc/types: 0.1.4
    typescript: 5.2.2

SWC Config:
    output: N/A
    .swcrc path: N/A

Next.js info:
    output: N/A

Expected behavior

With tsc:

function someDecorator(...) {
  console.log(context.metadata) // [Object: null prototype] {}
}

Actual behavior

With swc:

function someDecorator(...) {
  console.log(context.metadata) // undefined
}

Version

1.3.82

Additional context

Maybe this could be helpful:
microsoft/TypeScript#55453

[arklanq]

I have investigated the issue a bit more and found out that the decorators' metadata doesn't work at all.
Metadata is not saved after modifications are made in decorator body, have a look at an example below:

type ClassTarget = abstract new (...args: unknown[]) => unknown;

function inject<Class extends ClassTarget>(
  target: Class,
  context: ClassDecoratorContext<Class>
): void {
  context.metadata = Object.create(null); // Manually initializing metadata object, the same way tsc does
  context.metadata.foo = 'bar'; // Seeding example data
}

@inject
class App { }

// tsc
console.debug(App[Symbol.metadata]); // [Object: null prototype] { "foo": "bar" }

// swc
console.debug(App[Symbol.metadata]); // null

[kdy1]

I think swc is emitting the expected output for your repro.

https://play.swc.rs/?version=1.3.85&code=H4sIAAAAAAAAA02QwU7EIBCG7zzF3BYSw95b22jWR%2FBm9jCF6aa6Dg1MdzXGd3eENTHhwPzwffxBPleCwxlLecZ8IoEBcCqSMQgwXcF67%2FWgdLDxG6crvxwdDOPf1BszbxxkSQwLv1KQ%2ByoD%2BhDiWP6rR2sApG67lt9pEBKLXr4lTxRSRkn50OJmG43r4JKWCF%2BNKOlMPtK0neyN9%2B8kGFHQ9bDfa71I88IUzbcxD62ZCbXZ47qqRj1zStZVI0Am2TLDbsK86zVRStfvSwKowFA%2FQ1HrjM6%2Bsr35AYrFObg%2BAQAA&config=H4sIAAAAAAAAA22QsQ6CQAyGd5%2Fi0lmNoIMx0cVFF0f3BopB4e7SVgMhvLsHKmLidO3%2FXdu%2FbSbGwFUS2JgmhCHxyEI85EGR2ipWQQGtPUnCuVeYfqhKhzIshAYtpcQxqmMJSPk%2BIrXFMk%2BOpXesP7B9PaCMVjLH5djC0PBMLLmznZl4EcezxRJ%2By6lSYovFgQpP%2FfyRNbgR%2BX2BIicsaWyuLweyj%2B8hFPlC2n0C61Iyu62J1vNo9R74d5Wu06R9AlfYLbtVAQAA

[arklanq]

I think swc is emitting the expected output for your repro.

https://play.swc.rs/?version=1.3.85&code=H4sIAAAAAAAAA02QwU7EIBCG7zzF3BYSw95b22jWR%2FBm9jCF6aa6Dg1MdzXGd3eENTHhwPzwffxBPleCwxlLecZ8IoEBcCqSMQgwXcF67%2FWgdLDxG6crvxwdDOPf1BszbxxkSQwLv1KQ%2ByoD%2BhDiWP6rR2sApG67lt9pEBKLXr4lTxRSRkn50OJmG43r4JKWCF%2BNKOlMPtK0neyN9%2B8kGFHQ9bDfa71I88IUzbcxD62ZCbXZ47qqRj1zStZVI0Am2TLDbsK86zVRStfvSwKowFA%2FQ1HrjM6%2Bsr35AYrFObg%2BAQAA&config=H4sIAAAAAAAAA22QsQ6CQAyGd5%2Fi0lmNoIMx0cVFF0f3BopB4e7SVgMhvLsHKmLidO3%2FXdu%2FbSbGwFUS2JgmhCHxyEI85EGR2ipWQQGtPUnCuVeYfqhKhzIshAYtpcQxqmMJSPk%2BIrXFMk%2BOpXesP7B9PaCMVjLH5djC0PBMLLmznZl4EcezxRJ%2By6lSYovFgQpP%2FfyRNbgR%2BX2BIicsaWyuLweyj%2B8hFPlC2n0C61Iyu62J1vNo9R74d5Wu06R9AlfYLbtVAQAA

Mmm... it does not for me. I have copy-pasted the code from output to a plain javascript file and when executed with Node.js got the same results. Still context.metadata remains undefined.

PS: I have also upgraded all related dependencies so all the tests I run are based on the latest versions of @swc/*.

-  "@swc/cli": "^0.1.62",
-  "@swc/core": "^1.3.82",
-  "@swc/helpers": "^0.5.1",
+  "@swc/cli": "^0.1.62",
+  "@swc/core": "^1.3.85",
+  "@swc/helpers": "^0.5.2",

[kdy1]

If so, please provide a minimal reproduction, ideally with the swc playground.

[kdy1]

Your code does not produce undefined.

What browser do you use?

[arklanq]

@kdy1 Ahh, you don't see the output, as I am using console.debug which is unsupported in a browser environment.

Let me start from scratch with (this time) valid and a bit more descriptive reproduction:

Issue reproduction / Github

https://github.com/arklanq/swc-issue-7957-reproduction

Issue reproduction / CodeSandbox

https://codesandbox.io/p/sandbox/crazy-mopsa-n9tgzv?welcome=true

Issue reproduction / SWC Playground

I cannot create valid issue reproduction on SWC Playground, because I need to import

Please make sure to read README.md and the comments left in the code carefully.

[kdy1]

Ah, I finally got what you mean.

But babel emits an identical output as SWC.

https://babeljs.io/repl#?browsers=&build=&builtIns=false&corejs=3.21&spec=false&loose=false&code_lz=C4TwDgpgBAwgNgQwM5ICoIE4HMLCgXigQCMlgMEBjPAOwgHcoAKAOjcyyQC4oBXGgNY0A9vRoBtALoBKAgD4-gkWIDcAKDUAzftQCWwmlF00AVhGoAeeMiRQIAD2AQaAE1vWU6bLjlM1UKGAOXB4PJAAafyhKAydHUMQUABFzYQpgNJhYh2ArRKQ5NWkeADdhXRcoAG8omJokYTgIFjhhLCY6uOAWAFtcBBcEIOkVKAB6McUXCE1jCBc1AF8NAAFjM2o1SnyoAEEwMGqNAM1hYSZZGoCAjFxeDEMAcmJMR_UA5aWNOrIiA4IoHRGPswBc1AgDixTucRmogA&debug=false&forceAllTransforms=false&modules=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=false&presets=stage-2%2Ctypescript&prettier=false&targets=&version=7.22.20&externalPlugins=&assumptions=%7B%7D

[kdy1]

I looked at the spec and seems like the babel team just didn't implement it yet.

https://github.com/tc39/proposal-decorators

I think that's because it is not worth the cost, but well... I'll reopen this issue for now.

[arklanq]

@kdy1 This is a fairly new feature that was introduced recently in TypeScript 5.2, so maybe Babel team is not rushing to implement it now. Anyway, it is a stage 3 proposal so we can be almost sure the feature will land soon in ECMA standard so we should not ignore it.

Some reference:

• TS 5.2 changelog
• Decorator metadata proposal

[kdy1]

There are enormous amount of tasks that has higher priority than 100% parity with the specification.

[arklanq]

Sure, I'll keep an eye on the issue then.

[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.