AI & Agent Dev Bug Sandbox Back to Radar

Minifier Inlines Comma-Assigned Helper Arrow Causing SyntaxError: Identifier 'B' Has Already Been Declared

A syntax error occurs during Next.js build when a module uses a comma-separated variable declaration followed by arrow function assignments. The minifier's variable mangling renames variables in a way that causes a duplicate identifier declaration, breaking the build. The issue can be temporarily resolved by disabling mangling.

highConfidence 85%Next.js Affected V16.3.1

Origin Analysis

The SWC minifier (or underlying minifier) incorrectly handles a comma expression that assigns arrow functions to previously declared variables. During the inlining and mangling phase, it reuses the same short identifier (e.g., 'b') for different variables within the same scope, resulting in a duplicate declaration SyntaxError.

1. Create a new Next.js app using `npx create-next-app@latest my-app --yes`. 2. Create a file `repro.js` in the root with the exact code provided in the issue (var join, run; join = (e, t) => e + t, run = (rows) => { rows.map((s) => join(s.g, s.r)); return rows.map((s) => (i) => join(s.g, i.l)); }; export { run };). 3. Import the module in `app/page.tsx` as shown. 4. Run `npm run build`. 5. Observe the ChunkLoadError with SyntaxError: Identifier 'b' has already been declared.

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

Disabling mangling increases the size of the generated JavaScript bundles because variable names are not shortened. The `experimental.swcMinifyDebugOptions` API is experimental and may change or be removed in future Next.js releases, so this fix should be re-evaluated after upgrading. If the issue persists or new errors appear, rollback by removing this config and using the CLI flag `--no-mangling` or pinning to a Next.js version that includes the fix. Concurrency and cross-platform behavior should be unaffected, but always test the build on your specific environment.