feat: allow passing custom swc configuration to swcPlugin

[Romakita]

Information

This PR allow passing custom swc configuration to swcPlugin to solves the following issues: #1291 #1104 #1311

I know this feature isn't aligned with the tsup philosophy to minimize the configuration, but there are some needs that legitimate to introduce swc option.

Maybe a better solution can be found based on this PR ;)

Usage

When you use legacy TypeScript decorator by enabling emitDecoratorMetadata in your tsconfig, tsup will automatically use SWC to transpile
decorators. In this case, you can give extra swc configuration in the tsup.config.ts file.

For example, if you have to define useDefineForClassFields, you can do that as follows:

import { defineConfig } from 'tsup'

export default defineConfig({
  entry: ['src/index.ts'],
  splitting: false,
  sourcemap: true,
  clean: true,
  swc: {
    jsc: {
      transform: {
        useDefineForClassFields: true
      }
    }
  }
})

Note: some SWC options cannot be configured:

{
  "parser": {
    "syntax": "typescript",
    "decorators": true
  },
  "transform": {
    "legacyDecorator": true,
    "decoratorMetadata": true
  },
  "keepClassNames": true,
  "target": "es2022"
}

You can also define a custom .swcrc configuration file. Just set swcrc to true
in tsup.config.ts to allow SWC plugin to discover automatically your custom swc config file.

import { defineConfig } from 'tsup'

export default defineConfig({
  entry: ['src/index.ts'],
  splitting: false,
  sourcemap: true,
  clean: true,
  swc: {
    swcrc: true
  }
})

[codesandbox]

Review or Edit in CodeSandbox

Open the branch in Web Editor • VS Code • Insiders

Open Preview

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
tsup	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 29, 2025 5:48am

[Romakita]

Hi @egoist

can you please, review this PR and tell me if it’s ok for you ;)
I’ll be happy to made any changes on this PR if it’s necessary to resolve all issue related swc configuration.

thank you!

[sonofmagic]

Very Good!

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/egoist/tsup@1313

commit: cb1bfed

[Romakita]

Thanks a lot @egoist! keep going your amazing work!