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Turbopack Production Build Eliminates Side Effects Of Pure-Annotated IIFE-Returned Function

Next.js 16.3.1 with Turbopack minifier incorrectly removes the invocation of a function returned by a `/* @__PURE__ */`-annotated self-invoking function expression (IIFE). The minifier extends the pure annotation's scope to the subsequent call of the returned function, eliminating necessary side effects such as `requestAnimationFrame` scheduling and callback execution. This causes production-only regressions in applications using patterns emitted by esbuild/tsup keepNames, notably affecting `@radix-ui/react-scroll-area`.

highConfidence 92%Next.js Affected V16.3.1

Origin Analysis

The Turbopack/SWC minifier treats the result of a call expression annotated with `/* @__PURE__ */` as side-effect-free and propagates this assumption to any later call of that result. In this pattern, the outer IIFE is marked pure, but the returned function (assigned to a variable and then invoked separately) has observable side effects. The minifier incorrectly considers the invocation of the returned function to be pure and removes it entirely.

1. Create a Next.js 16.3.1 project with Turbopack enabled by default. 2. Use a `/* @__PURE__ */`-annotated IIFE that returns a named function via a name-preserving helper (e.g., `__name`). Call that returned function inside a `useEffect` with side effects (e.g., starting a `requestAnimationFrame` loop). 3. Run `npm run build && npm run start`. Observe that the side effects never occur (e.g., counter stays at 0). 4. Run `npm run dev` and observe that side effects work correctly. 5. Inspect the production chunk to see the call to the returned function compiled away to a no-op.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This workaround may slightly reduce tree-shaking effectiveness for the affected code, potentially increasing bundle size. It does not address the underlying minifier bug and may need to be reapplied after updates to the affected package or Next.js. If applying to `node_modules`, the patch will be lost on reinstall; use a persistent patch mechanism (e.g., `patch-package`). Rollback is straightforward: restore the original annotations. Official fix should be implemented in Turbopack's minifier to correctly limit the scope of pure annotations.