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Modules: TypeScript

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:

    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

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 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:

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. 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.