> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt.

# Use TypeScript

Rslib supports TypeScript by default, allowing you to directly use `.ts` and `.tsx` files in your projects.

## TypeScript transpilation

Rslib uses SWC by default for transpiling TypeScript code, and it also supports switching to Babel for transpilation.

### isolatedModules

Unlike the native TypeScript compiler, tools like SWC and Babel compile each file separately and cannot determine whether an imported name is a type or a value. Therefore, when using TypeScript in Rslib, you need to enable the [isolatedModules](https://typescriptlang.org/tsconfig/#isolatedModules) option in your `tsconfig.json` file:

```json title="tsconfig.json"
{
  "compilerOptions": {
    "isolatedModules": true
  }
}
```

This option can help you avoid using certain syntax that cannot be correctly compiled by SWC and Babel, such as cross-file type references. It will guide you to correct the corresponding usage:

```ts
// Wrong
export { SomeType } from './types';

// Correct
export type { SomeType } from './types';
```

> See [SWC - Migrating from tsc](https://swc.rs/docs/migrating-from-tsc) for more details about the differences between SWC and tsc.

## Preset types

`@rslib/core` provides some preset type definitions, including CSS Modules, static assets, `import.meta` and other types.

Add the preset types to [`compilerOptions.types`](https://www.typescriptlang.org/tsconfig/#types) in `tsconfig.json`:

```json title="tsconfig.json"
{
  "compilerOptions": {
    "types": ["@rslib/core/types"]
  }
}
```

If your project already has `compilerOptions.types`, append `@rslib/core/types` to the existing list.

> Rslib's preset types are the same as Rsbuild's preset types. See Rsbuild's [types.d.ts](https://github.com/web-infra-dev/rsbuild/blob/main/packages/core/types.d.ts) for the complete type definitions.

## Type checking

When transpiling TypeScript code using tools like SWC and Babel, type checking is not performed.

Rslib provides the [lib.dts](/config/lib/dts.md) configuration item for generating TypeScript declaration files, and type checking is performed by default during the generation process.

You can skip type checking by setting the [noCheck](https://www.typescriptlang.org/tsconfig/#noCheck) configuration item to `true` in the `tsconfig.json` file.

## tsconfig.json path

Rslib by default reads the `tsconfig.json` file from the root directory. You can use [source.tsconfigPath](/config/rsbuild/source.md#sourcetsconfigpath) to configure a custom `tsconfig.json` file path.

```ts title="rslib.config.ts"
export default {
  lib: [
    // ...
  ],
  source: {
    tsconfigPath: './tsconfig.custom.json',
  },
};
```

## Decorators version

Rslib uses the [`2023-11`](https://rsbuild.rs/config/source/decorators#2023-11) version of the decorators by default.

If [experimentalDecorators](https://www.typescriptlang.org/tsconfig/#experimentalDecorators) is enabled in `tsconfig.json`, Rslib will set [source.decorators.version](/config/rsbuild/source.md#sourcedecorators) to `legacy` to use the legacy decorators.