esm-loader

Node.js import hook to instantaneously transform TypeScript to ESM on demand using esbuild.

Features

• Transforms TypeScript to ESM on demand
• Classic Node.js resolution (extensionless & directory imports)
• Cached for performance boost
• Supports Node.js v12.20.0+
• Handles node: import prefixes
• Resolves tsconfig.json paths

Tip:

esm-loader doesn't hook into require() calls or transform CommonJS files (.js in commonjs package, .cjs, .cts).

Use this with cjs-loader for CommonJS support. Alternatively, use tsx to handle them both automatically.

Install

npm install --save-dev @esbuild-kit/esm-loader

Usage

Pass @esbuild-kit/esm-loader into the --loader flag.

node --loader @esbuild-kit/esm-loader ./file.ts

TypeScript configuration

The following properties are used from tsconfig.json in the working directory:

• strict: Whether to transform to strict mode
• jsx: Whether to transform JSX

  Warning: When set to preserve, the JSX syntax will remain untransformed. To prevent Node.js from throwing a syntax error, chain another Node.js loader that can transform JSX to JS.

• jsxFactory: How to transform JSX
• jsxFragmentFactory: How to transform JSX Fragments
• jsxImportSource: Where to import JSX functions from
• allowJs: Whether to apply the tsconfig to JS files
• paths: For resolving aliases

Custom tsconfig.json path

By default, tsconfig.json will be detected from the current working directory.

To set a custom path, use the ESBK_TSCONFIG_PATH environment variable:

ESBK_TSCONFIG_PATH=./path/to/tsconfig.custom.json node --loader @esbuild-kit/esm-loader ./file.ts

Cache

Modules transformations are cached in the system cache directory (TMPDIR). Transforms are cached by content hash so duplicate dependencies are not re-transformed.

Set environment variable ESBK_DISABLE_CACHE to a truthy value to disable the cache:

ESBK_DISABLE_CACHE=1 node --loader @esbuild-kit/esm-loader ./file.ts

FAQ

Can it import JSON modules?

Yes. This loader transpiles JSON modules so it's also compatible with named imports.

Can it import ESM modules over network?

Node.js has built-in support for network imports behind the --experimental-network-imports flag.

You can pass it in with esm-loader:

node --loader @esbuild-kit/esm-loader --experimental-network-imports ./file.ts

Can it resolve files without an extension?

In ESM, import paths must be explicit (must include file name and extension).

For backwards compatibility, this loader adds support for classic Node resolution for extensions: .js, .json, .ts, .tsx, .jsx. Resolving a index file by the directory name works too.

import file from './file' // -> ./file.js
import directory from './directory' // -> ./directory/index.js

Can it use Node.js's CommonJS resolution algorithm?

ESM import resolution expects explicit import paths, whereas CommonJS resolution expects implicit imports (eg. extensionless & directory imports).

As a result of this change, Node.js changes how it imports a path that matches both a file and directory. In ESM, the directory would be imported, but in CJS, the file would be imported.

To use to the CommonJS resolution algorithm, use the --experimental-specifier-resolution=node flag.

node --loader @esbuild-kit/esm-loader --experimental-specifier-resolution=node ./file.ts

Related

• tsx - Node.js runtime powered by esbuild using @esbuild-kit/cjs-loader and @esbuild-kit/esm-loader.

• @esbuild-kit/cjs-loader - TypeScript & ESM to CJS transpiler using the Node.js loader API.