egoist · egoist · May 16, 2025 · Mar 4, 2025 · Mar 4, 2025
diff --git a/docs/README.md b/docs/README.md
@@ -609,6 +609,64 @@ tsup --tsconfig tsconfig.prod.json
 
 By default, tsup try to find the `tsconfig.json` file in the current directory, if it's not found, it will use the default tsup config.
 
+### Using custom Swc configuration
+
+When you use legacy TypeScript decorator by enabling `emitDecoratorMetadata` in your tsconfig, tsup will automatically use [SWC](https://swc.rs) 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:
+```ts
+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:
+
+```json
+{
+  "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.
+
+```ts
+import { defineConfig } from 'tsup'
+
+export default defineConfig({
+  entry: ['src/index.ts'],
+  splitting: false,
+  sourcemap: true,
+  clean: true,
+  swc: {
+    swcrc: true
+  }
+})
+```
+
 ## Troubleshooting
 
 ### error: No matching export in "xxx.ts" for import "xxx"

diff --git a/schema.json b/schema.json
@@ -181,6 +181,9 @@
             }
           ]
         },
+        "swc": {
+          "type": "object"
+        },
         "globalName": {
           "type": "string"
         },

diff --git a/src/esbuild/index.ts b/src/esbuild/index.ts
@@ -138,7 +138,7 @@ export async function runEsbuild(
         skipNodeModulesBundle: options.skipNodeModulesBundle,
         tsconfigResolvePaths: options.tsconfigResolvePaths,
       }),
-    options.tsconfigDecoratorMetadata && swcPlugin({ logger }),
+    options.tsconfigDecoratorMetadata && swcPlugin({ ...options.swc, logger }),
     nativeNodeModulesPlugin(),
     postcssPlugin({
       css,

diff --git a/src/esbuild/swc.test.ts b/src/esbuild/swc.test.ts
@@ -0,0 +1,101 @@
+import { describe, expect, test, vi } from 'vitest'
+import { swcPlugin, type SwcPluginConfig } from './swc'
+import { localRequire } from '../utils'
+
+vi.mock('../utils')
+
+const getFixture = async (opts: Partial<SwcPluginConfig> = {}) => {
+  const swc = {
+    transformFile: vi.fn().mockResolvedValue({
+      code: 'source-code',
+      map: JSON.stringify({
+        sources: ['file:///path/to/file.ts'],
+      }),
+    }),
+  }
+
+  const logger = {
+    warn: vi.fn(),
+    error: vi.fn(),
+    info: vi.fn(),
+  }
+
+  const build = {
+    initialOptions: {
+      keepNames: true,
+    },
+    onLoad: vi.fn(),
+  }
+
+  vi.mocked(localRequire).mockReturnValue(swc)
+
+  const plugin = swcPlugin({
+    ...opts,
+    logger: logger as never,
+  })
+
+  await plugin.setup(build as never)
+
+  const onLoad = build.onLoad.mock.calls[0][1] as Function
+
+  return { swc, onLoad, logger, build }
+}
+describe('swcPlugin', () => {
+  test('swcPlugin transforms TypeScript code with decorators and default plugin swc option', async () => {
+    const { swc, onLoad } = await getFixture()
+
+    await onLoad({
+      path: 'file.ts',
+    })
+
+    expect(swc.transformFile).toHaveBeenCalledWith('file.ts', {
+      configFile: false,
+      jsc: {
+        keepClassNames: true,
+        parser: {
+          decorators: true,
+          syntax: 'typescript',
+        },
+        target: 'es2022',
+        transform: {
+          decoratorMetadata: true,
+          legacyDecorator: true,
+        },
+      },
+      sourceMaps: true,
+      swcrc: false,
+    })
+  })
+  test('swcPlugin transforms TypeScript code and use given plugin swc option', async () => {
+    const { swc, onLoad } = await getFixture({
+      jsc: {
+        transform: {
+          useDefineForClassFields: true,
+        },
+      },
+    })
+
+    await onLoad({
+      path: 'file.ts',
+    })
+
+    expect(swc.transformFile).toHaveBeenCalledWith('file.ts', {
+      configFile: false,
+      jsc: {
+        keepClassNames: true,
+        parser: {
+          decorators: true,
+          syntax: 'typescript',
+        },
+        target: 'es2022',
+        transform: {
+          decoratorMetadata: true,
+          legacyDecorator: true,
+          useDefineForClassFields: true,
+        },
+      },
+      sourceMaps: true,
+      swcrc: false,
+    })
+  })
+})
diff --git a/src/esbuild/swc.ts b/src/esbuild/swc.ts
@@ -3,11 +3,13 @@
  */
 import path from 'node:path'
 import { localRequire } from '../utils'
-import type { JscConfig } from '@swc/core'
+import type { JscConfig, Options } from '@swc/core'
 import type { Plugin } from 'esbuild'
 import type { Logger } from '../log'
 
-export const swcPlugin = ({ logger }: { logger: Logger }): Plugin => {
+export type SwcPluginConfig = { logger: Logger } & Options
+
+export const swcPlugin = ({ logger, ...swcOptions }: SwcPluginConfig): Plugin => {
   return {
     name: 'swc',
 
@@ -29,11 +31,14 @@ export const swcPlugin = ({ logger }: { logger: Logger }): Plugin => {
         const isTs = /\.tsx?$/.test(args.path)
 
         const jsc: JscConfig = {
+          ...swcOptions.jsc,
           parser: {
+            ...swcOptions.jsc?.parser,
             syntax: isTs ? 'typescript' : 'ecmascript',
             decorators: true,
           },
           transform: {
+            ...swcOptions.jsc?.transform,
             legacyDecorator: true,
             decoratorMetadata: true,
           },
@@ -42,10 +47,11 @@ export const swcPlugin = ({ logger }: { logger: Logger }): Plugin => {
         }
 
         const result = await swc.transformFile(args.path, {
+          ...swcOptions,
           jsc,
           sourceMaps: true,
           configFile: false,
-          swcrc: false,
+          swcrc: swcOptions.swcrc ?? false,
         })
 
         let code = result.code

diff --git a/src/options.ts b/src/options.ts
@@ -4,6 +4,7 @@ import type { MinifyOptions } from 'terser'
 import type { MarkRequired } from 'ts-essentials'
 import type { Plugin } from './plugin'
 import type { TreeshakingStrategy } from './plugins/tree-shaking'
+import type { SwcPluginConfig } from './esbuild/swc.js'
 
 export type KILL_SIGNAL = 'SIGKILL' | 'SIGTERM'
 
@@ -256,6 +257,8 @@ export type Options = {
    * @default true
    */
   removeNodeProtocol?: boolean
+
+  swc?: SwcPluginConfig;
 }
 
 export interface NormalizedExperimentalDtsConfig {
@@ -272,4 +275,5 @@ export type NormalizedOptions = Omit<
   tsconfigResolvePaths: Record<string, string[]>
   tsconfigDecoratorMetadata?: boolean
   format: Format[]
+  swc?: SwcPluginConfig
 }
diff --git a/vitest.config.mts b/vitest.config.mts
@@ -4,5 +4,6 @@ export default defineConfig({
   test: {
     testTimeout: 50000,
     globalSetup: 'vitest-global.ts',
+    include: ["test/*.test.ts", "src/**/*.test.ts"]
   },
 })