# Modules: TypeScript

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> Stability: 1.0 - Early development

## 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`:

   ```bash
   npm install --save-dev tsx
   ```

2. Then you can run your TypeScript code via:

   ```bash
   npx tsx your-file.ts
   ```

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

   ```bash
   node --import=tsx your-file.ts
   ```

## Type stripping

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> Stability: 1.0 - Early development

The flag [`--experimental-strip-types`][] enables Node.js to run TypeScript
files. By default Node.js will execute only files that 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.
To enable the transformation of such features
use the flag [`--experimental-transform-types`][].
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 full TypeScript support, 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.

### TypeScript features

Since Node.js is only removing inline types, any TypeScript features that
involve _replacing_ TypeScript syntax with new JavaScript syntax will error,
unless the flag [`--experimental-transform-types`][] is passed.

The most prominent features that require transformation are:

* `Enum`
* `namespaces`
* `legacy module`
* parameter properties

Since Decorators are currently a [TC39 Stage 3 proposal](https://github.com/tc39/proposal-decorators)
and will soon be supported by the JavaScript engine,
they are not transformed and will result in a parser error.
This is a temporary limitation and will be resolved in the future.

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:

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

This will result in a runtime error:

```ts
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.
When [`--experimental-transform-types`][] is enabled, source-maps
are enabled by default.

### 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.

[CommonJS]: modules.md
[ES Modules]: esm.md
[Full TypeScript support]: #full-typescript-support
[`--experimental-strip-types`]: cli.md#--experimental-strip-types
[`--experimental-transform-types`]: cli.md#--experimental-transform-types
[`tsx`]: https://tsx.is/
[`verbatimModuleSyntax`]: https://www.typescriptlang.org/tsconfig/#verbatimModuleSyntax
[file extensions are mandatory]: esm.md#mandatory-file-extensions
[full support]: #full-typescript-support
[the same way as `.js` files.]: packages.md#determining-module-system
[type stripping]: #type-stripping
