Turning your TypeScript package into "type": "module"

This repository contains an example TypeScript package with full ES Module support.

Import specifier

Unlike CommonJS Modules, file extensions (.js) are mandatory for ES Modules in Node.js.

This is more problematic in TypeScript; source extensions and output extensions differ in the language.

// Which declaration should we use to import foo.ts?
import { foo } from "foo";
import { foo } from "foo.js";
import { foo } from "foo.ts";

In this repository, we use foo.js to import foo.ts because both TypeScript and Node.js supports it.

# TypeScript's error for an attempt to import .ts files
src/index.ts:1:21 - error TS2691: An import path cannot end with a '.ts' extension. Consider importing './square' instead.

1 import { square } from "./square.ts";
                         ~~~~~~~~~~~~~


Found 1 error.

TypeScript

As stated above, TypeScript can resolve *.js imports as *.ts or *.tsx files.

// index.ts
import { square } from "./square.js"; // resolves to square.ts

When transpiled, square.ts turns into square.js so Node.js straightforwardly resolves the import.

Problems with react-jsx

When "jsx": "react-jsx" is specified, tsc produces the following snippet:

import { jsx as _jsx } from "react/jsx-runtime";

The import is expected to be resolved as node_modules/react/jsx-runtime.js. As of the current version (React v17.0.1), it fails due to extension mismatch.

internal/process/esm_loader.js:74
    internalBinding('errors').triggerUncaughtException(
                              ^

Error [ERR_MODULE_NOT_FOUND]: Cannot find module '$CWD/node_modules/react/jsx-runtime' imported from $CWD/dist-ts/App.js
Did you mean to import react/jsx-runtime.js?
    at finalizeResolution (internal/modules/esm/resolve.js:276:11)
    at moduleResolve (internal/modules/esm/resolve.js:699:10)
    at Loader.defaultResolve [as _resolve] (internal/modules/esm/resolve.js:810:11)
    at Loader.resolve (internal/modules/esm/loader.js:86:40)
    at Loader.getModuleJob (internal/modules/esm/loader.js:230:28)
    at ModuleWrap.<anonymous> (internal/modules/esm/module_job.js:56:40)
    at link (internal/modules/esm/module_job.js:55:36) {
  code: 'ERR_MODULE_NOT_FOUND'
}

The fix https://github.com/facebook/react/pull/20304 will land in the next version of React.

Webpack

Due to Webpack's pipeline structure, imports must be resolved to the source extension (.ts), not the output extension (.js).

Webpack has the ability to automatically add extensions, but unfortunately, there's no removal counterpart.

asset index.js 2.42 KiB [emitted] (name: index)
runtime modules 274 bytes 1 module
./index.ts 194 bytes [built] [code generated]

ERROR in ./index.ts 1:0-31
Module not found: Error: Can't resolve './square.js' in '$CWD/src'
resolve './square.js' in '$CWD/src'
  using description file: $CWD/package.json (relative path: ./src)
    using description file: $CWD/package.json (relative path: ./src/square.js)
      no extension
        $CWD/src/square.js doesn't exist
      .wasm
        $CWD/src/square.js.wasm doesn't exist
      .mjs
        $CWD/src/square.js.mjs doesn't exist
      .js
        $CWD/src/square.js.js doesn't exist
      .jsx
        $CWD/src/square.js.jsx doesn't exist
      .ts
        $CWD/src/square.js.ts doesn't exist
      .tsx
        $CWD/src/square.js.tsx doesn't exist
      .json
        $CWD/src/square.js.json doesn't exist
      as directory
        $CWD/src/square.js doesn't exist

In this repository, we remove the .js extension using a babel plugin. We should enable this plugin only for Webpack.

ts-node

As of ts-node 9.1.1, you cannot use the ts-node command in an ESM package:

TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts" for $CWD/src/index.ts
    at Loader.defaultGetFormat [as _getFormat] (internal/modules/esm/get_format.js:71:15)
    at Loader.getFormat (internal/modules/esm/loader.js:102:42)
    at Loader.getModuleJob (internal/modules/esm/loader.js:231:31)
    at Loader.import (internal/modules/esm/loader.js:165:17)
    at Object.loadESM (internal/process/esm_loader.js:68:5)

You need a different command to enable the experimental ESM support:

$ node --loader ts-node/esm src/index.js

Jest (with babel-jest)

You need to pass --experimental-vm-modules to Node.js, but that's not enough.

$ node --experimental-vm-modules node_modules/.bin/jest --config=jest-babel.config.ts
(node:27619) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
(node:27660) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
(node:27654) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
 FAIL  src/square.test.ts
   Test suite failed to run

    Jest encountered an unexpected token

    This usually means that you are trying to import a file which Jest cannot parse, e.g. it's not plain JavaScript.

    By default, if Jest sees a Babel config, it will use that to transform your files, ignoring "node_modules".

    Here's what you can do:
      If you are trying to use ECMAScript Modules, see https://jestjs.io/docs/en/ecmascript-modules for how to enable it.
      To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
      If you need a custom transformation specify a "transform" option in your config.
      If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.

    You'll find more details and examples of these config options in the docs:
    https://jestjs.io/docs/en/configuration.html

    Details:

    $CWD/src/square.test.ts:1
    ({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){import { square } from "./square";
                                                                                             ^^^^^^

    SyntaxError: Cannot use import statement outside a module

      at Runtime.createScriptFromCode (node_modules/jest-runtime/build/index.js:1350:14)

Test Suites: 1 failed, 1 total
Tests:       0 total
Snapshots:   0 total
Time:        0.803 s
Ran all test suites.
error Command failed with exit code 1.

You need to set extensionsToTreatAsEsm to [".ts"], but the option is only available in the coming Jest 27.

Translating extensions

By default, Jest only tries foo.js, foo.js.js or foo.js.ts for foo.js request.

(node:939) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
 PASS  src/square.test.ts
 FAIL  src/square42.test.ts
   Test suite failed to run

    Error [ERR_VM_MODULE_LINKING_ERRORED]: Linking has already failed for the provided module

          at async Promise.all (index 0)

Test Suites: 1 failed, 1 passed, 2 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.286 s, estimated 1 s
Ran all test suites.

(node:1052) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
 FAIL  src/square.test.ts
   Test suite failed to run

    Cannot find module './square.js' from 'square.test.ts'

      at Resolver.resolveModule (../node_modules/jest-resolve/build/index.js:311:11)

 FAIL  src/square42.test.ts
   Test suite failed to run

    Cannot find module './square42.js' from 'square42.test.ts'

      at Resolver.resolveModule (../node_modules/jest-resolve/build/index.js:311:11)

Test Suites: 2 failed, 2 total
Tests:       0 total
Snapshots:   0 total
Time:        0.288 s, estimated 1 s
Ran all test suites.
error Command failed with exit code 1.

To resolve foo.js as foo.ts, you can use moduleNameMapper to strip the .js extension.

export default {
  // ...
  moduleNameMapper: {
    "^(.*)\\.js$": "$1",
  },
  // ...
};

Jest (with ts-jest)

For whatever reason, it seems ts-jest just works with Jest 26.

However, this is no longer the case with Jest 27, which comes with the proper support for ES Modules. See the previous section below.

Note that, you'll need the following additional configuration:

  globals: {
    'ts-jest': {
      useESM: true,
    },
  },

Otherwise you may (conditionally) see errors like below:

(node:25435) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
(node:25476) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
(node:25470) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
 FAIL  src/square42.test.ts
   Test suite failed to run

    ReferenceError: exports is not defined

      1 | import { square42 } from "./square42.js";
    > 2 |
        | ^
      3 | describe("square42", () => {
      4 |   it("returns 1764", () => {
      5 |     expect(square42()).toBe(1764);

      at square42.test.ts:2:23

 FAIL  src/square.test.ts
   Test suite failed to run

    ReferenceError: exports is not defined

      1 | import { square } from "./square.js";
    > 2 |
        | ^
      3 | describe("square", () => {
      4 |   it("squares the number", () => {
      5 |     expect(square(42)).toBe(1764);

      at square.test.ts:2:23

Test Suites: 2 failed, 2 total
Tests:       0 total
Snapshots:   0 total
Time:        4.113 s
Ran all test suites.