Question 1

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

Question 2

What is `isolatedDeclarations` and how does it enable single-file transpilation for declaration emit?

Accepted Answer

**Target Audience:** Developers who build or publish TypeScript libraries or large monorepos and want to understand why `.d.ts` generation is slow and what `isolatedDeclarations` changes.
**Difficulty:** Hard
**TypeScript version:** `isolatedDeclarations` arrived in TypeScript 5.5. The page compiler is TypeScript 5.4.5, which rejects the flag (`error TS5023: Unknown compiler option '--isolatedDeclarations'.`, checked here), so the page examples only show the underlying idea. Every `isolatedDecla

TYPESCRIPTDECODED.

Hard

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

What is `isolatedDeclarations` and how does it enable single-file transpilation for declaration emit?