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nerminamiller-postman merged 2 commits into from
Sep 16, 2026
Merged

Document esmOnly option for the TypeScript SDK generator #6942

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docs(typescript): document esmOnly generator option
devin-ai-integration[bot] Sep 13, 2026
a190fb4
Merge origin/main into devin/1789319077-ts-esm-only
devin-ai-integration[bot] Sep 16, 2026
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16 changes: 16 additions & 0 deletions fern/products/sdks/generators/typescript/configuration.mdx
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Expand Up @@ -88,6 +88,22 @@ const bar: MyRootType.Foo.Bar = {};
```
</ParamField>

<ParamField path="esmOnly" type="boolean" default="false" toc={true}>
Ships only the ECMAScript module (ESM) build of the generated package. By default, the generator publishes both CommonJS and ESM builds, which exposes consumers to the [dual package hazard](https://github.com/GeoffreyBooth/dual-package-hazard). When enabled, `package.json` sets `"type": "module"`, `main`, `module`, and `types` point at `./dist/esm/index.mjs` and `./dist/esm/index.d.mts`, the `exports` map (including subpackage exports) has no `require` conditions, and no `tsconfig.cjs.json` or `build:cjs` script is generated.
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`esmOnly` can't be combined with `useLegacyExports: true` or `bundle: true`; generation fails with an error.

```yml title="generators.yml"
groups:
ts-sdk:
generators:
- name: fern-typescript-sdk
version: <Markdown src="/snippets/version-number-ts.mdx"/>
config:
esmOnly: true
```
</ParamField>

<ParamField path="exactOptionalPropertyTypes" type="boolean" default="false" toc={true}>
When enabled, every optional property in the generated SDK is emitted as `prop?: T | undefined`, and the generated `tsconfig` files enable TypeScript's [`exactOptionalPropertyTypes`](https://www.typescriptlang.org/tsconfig/#exactOptionalPropertyTypes) compiler option. This covers model types, inlined request types, client options, errors, and the `core/` utilities, so consumers whose own `tsconfig` sets `exactOptionalPropertyTypes: true` can use the SDK without type errors.

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