Table of contents

- Modules: TypeScript
  - Enabling
  - Full TypeScript support
  - Type stripping
    - Determining module system
    - Unsupported TypeScript features
    - Importing types without `type` keyword
    - Non-file forms of input
    - Source maps
    - Type stripping in dependencies

## Modules: TypeScript#

### Enabling#

There are two ways to enable runtime TypeScript support in Node.js:

1. For full support of all of TypeScript's syntax and features, including using any version of TypeScript, use a third-party package.

2. For lightweight support, you can use the built-in support for type stripping.

### Full TypeScript support#

To use TypeScript with full support for all TypeScript features, including `tsconfig.json`, you can use a third-party package. These instructions use `tsx` as an example but there are many other similar libraries available.

1. Install the package as a development dependency using whatever package manager you're using for your project. For example, with `npm`:

       npm install --save-dev tsx

2. Then you can run your TypeScript code via:

       npx tsx your-file.ts

   Or alternatively, you can run with `node` via:

       node --import=tsx your-file.ts

### Type stripping#

Added in: REPLACEME

Stability: 1.0 - Early development

The flag `--experimental-strip-types` enables Node.js to run TypeScript files that contain only type annotations. Such files contain no TypeScript features that require transformation, such as enums or namespaces. Node.js will replace inline type annotations with whitespace, and no type checking is performed. TypeScript features that depend on settings within `tsconfig.json`, such as paths or converting newer JavaScript syntax to older standards, are intentionally unsupported. To get fuller TypeScript support, including support for enums and namespaces and paths, see Full TypeScript support.

The type stripping feature is designed to be lightweight. By intentionally not supporting syntaxes that require JavaScript code generation, and by replacing inline types with whitespace, Node.js can run TypeScript code without the need for source maps.

#### Determining module system#

Node.js supports both CommonJS and ES Modules syntax in TypeScript files. Node.js will not convert from one module system to another; if you want your code to run as an ES module, you must use `import` and `export` syntax, and if you want your code to run as CommonJS you must use `require` and `module.exports`.

- `.ts` files will have their module system determined the same way as `.js` files. To use `import` and `export` syntax, add `"type": "module"` to the nearest parent `package.json`.
- `.mts` files will always be run as ES modules, similar to `.mjs` files.
- `.cts` files will always be run as CommonJS modules, similar to `.cjs` files.
- `.tsx` files are unsupported.

As in JavaScript files, file extensions are mandatory in `import` statements and `import()` expressions: `import './file.ts'`, not `import './file'`. Because of backward compatibility, file extensions are also mandatory in `require()` calls: `require('./file.ts')`, not `require('./file')`, similar to how the `.cjs` extension is mandatory in `require` calls in CommonJS files.

The `tsconfig.json` option `allowImportingTsExtensions` will allow the TypeScript compiler `tsc` to type-check files with `import` specifiers that include the `.ts` extension.

#### Unsupported TypeScript features#

Since Node.js is only removing inline types, any TypeScript features that involve replacing TypeScript syntax with new JavaScript syntax will error. This is by design. To run TypeScript with such features, see Full TypeScript support.

The most prominent unsupported features that require transformation are:

- `Enum`
- `experimentalDecorators`
- `namespaces`
- parameter properties

In addition, Node.js does not read `tsconfig.json` files and does not support features that depend on settings within `tsconfig.json`, such as paths or converting newer JavaScript syntax into older standards.

#### Importing types without `type` keyword#

Due to the nature of type stripping, the `type` keyword is necessary to correctly strip type imports. Without the `type` keyword, Node.js will treat the import as a value import, which will result in a runtime error. The tsconfig option `verbatimModuleSyntax` can be used to match this behavior.

This example will work correctly:

    import type { Type1, Type2 } from './module.ts';
    import { fn, type FnParams } from './fn.ts';

This will result in a runtime error:

    import { Type1, Type2 } from './module.ts';
    import { fn, FnParams } from './fn.ts';

#### Non-file forms of input#

Type stripping can be enabled for `--eval`. The module system will be determined by `--input-type`, as it is for JavaScript.

TypeScript syntax is unsupported in the REPL, STDIN input, `--print`, `--check`, and `inspect`.

#### Source maps#

Since inline types are replaced by whitespace, source maps are unnecessary for correct line numbers in stack traces; and Node.js does not generate them. For source maps support, see Full TypeScript support.

#### Type stripping in dependencies#

To discourage package authors from publishing packages written in TypeScript, Node.js will by default refuse to handle TypeScript files inside folders under a `node_modules` path.