node/doc/api/typescript.md

Modules: TypeScript

Stability: 2 - Stable

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

By default Node.js will execute TypeScript files that contains only erasable TypeScript syntax. Node.js will replace TypeScript syntax with whitespace, and no type checking is performed. To disable this feature, use the flag --no-strip-types.

Node.js ignores tsconfig.json files and therefore 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.

Type stripping is compatible with most versions of TypeScript but we recommend version 5.8 or newer with the following tsconfig.json settings:

{
  "compilerOptions": {
     "noEmit": true, // Optional - see note below
     "target": "esnext",
     "module": "nodenext",
     "rewriteRelativeImportExtensions": true,
     "erasableSyntaxOnly": true,
     "verbatimModuleSyntax": true
  }
}

Use the noEmit option if you intend to only execute *.ts files, for example a build script. You won't need this flag if you intend to distribute *.js files.

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.

The most prominent features that require transformation are:

• Enum declarations
• namespace with runtime code
• parameter properties
• import aliases

namespaces that do not contain runtime code are supported. This example will work correctly:

// This namespace is exporting a type
namespace TypeOnly {
   export type A = string;
}

This will result in ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX error:

// This namespace is exporting a value
namespace A {
   export let x = 1
}

Since Decorators are currently a TC39 Stage 3 proposal, they are not transformed and will result in a parser error. Node.js does not provide polyfills and thus will not support decorators until they are supported natively in JavaScript.

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 and STDIN. The module system will be determined by --input-type, as it is for JavaScript.

TypeScript syntax is unsupported in the REPL, --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.

Type stripping in dependencies

To discourage package authors from publishing packages written in TypeScript, Node.js refuses to handle TypeScript files inside folders under a node_modules path.

Paths aliases

tsconfig "paths" won't be transformed and therefore produce an error. The closest feature available is subpath imports with the limitation that they need to start with #.