Question 1

What does `arethetypeswrong` (attw) detect and how do you fix false ESM/CJS packages?

Accepted Answer

**Target Audience:** Library authors who publish to npm, and app developers who hit a strange type error when importing a package that runs fine.
**Difficulty:** Medium
**TypeScript version:** The page examples were checked with TypeScript 5.4.5 (`--strict`). The package reproductions in sections 4 to 7 are small multi-file projects compiled with TypeScript 5.4.5 (and 5.9.3 where noted, `module` / `moduleResolution` `nodenext`) and run with Node.js 22.22. attw itself was **not** run here (it is

Question 2

How do you author library types that pass `isolatedDeclarations` for `oxc` / `tsdown` builds?

Accepted Answer

**Target Audience:** Library authors moving their build to tsdown, Rolldown or another oxc-based tool, and anyone asked to make an existing codebase pass `isolatedDeclarations`.
**Difficulty:** Hard
**TypeScript version:** The page examples were checked with TypeScript 5.4.5 `--strict` (which has no `isolatedDeclarations` flag). Every `isolatedDeclarations` error and `.d.ts` file below came from TypeScript 5.9.3 and from oxc's isolated-declaration transform as bundled in rolldown 1.0.3 (`isolate

TYPESCRIPTDECODED.

Medium

What does `arethetypeswrong` (attw) detect and how do you fix false ESM/CJS packages?

Hard

How do you author library types that pass `isolatedDeclarations` for `oxc` / `tsdown` builds?