fix(esbuild): enable experimentalDecorators by default

packages/vite/src/node/plugins/esbuild.ts

@@ -142,6 +142,14 @@ export async function transformWithEsbuild(
       compilerOptions.useDefineForClassFields = false
     }
 
+    // esbuild v0.18 only transforms decorators when `experimentalDecorators` is set to `true`.
+    // To preserve compat with the esbuild breaking change, we set `experimentalDecorators` to
+    // `true` by default if it's unset.
+    // TODO: Remove this in Vite 5
+    if (compilerOptions.experimentalDecorators === undefined) {
+      compilerOptions.experimentalDecorators = true
+    }
+
     // esbuild uses tsconfig fields when both the normal options and tsconfig was set
     // but we want to prioritize the normal options
     if (options) {

playground/tsconfig-json/__tests__/tsconfig-json.spec.ts

@@ -67,4 +67,16 @@ describe('transformWithEsbuild', () => {
       'import { MainTypeOnlyClass } from "./not-used-type";',
     )
   })
+
+  test('experimentalDecorators', async () => {
+    const main = path.resolve(__dirname, '../src/decorator.ts')
+    const mainContent = fs.readFileSync(main, 'utf-8')
+    // Should not error when transpiling decorators
+    // TODO: In Vite 5, this should require setting `tsconfigRaw.experimentalDecorators`
+    // or via the closest `tsconfig.json`
+    const result = await transformWithEsbuild(mainContent, main, {
+      target: 'es2020',
+    })
+    expect(result.code).toContain('__decorateClass')
+  })
 })

playground/tsconfig-json/src/decorator.ts

@@ -0,0 +1,8 @@
+function first() {
+  return function (...args: any[]) {}
+}
+
+export class Foo {
+  @first()
+  method() {}
+}