[Bug]: Warning: Failed to load the ES module: jest.config.cts

[jlowcs]

Version

30.2.0

Steps to reproduce

1. create a simple repo with this package.json:

{
  "name": "test-jest",
  "version": "1.0.0",
  "devDependencies": {
    "@swc/jest": "^0.2.39",
    "@types/jest": "^30.0.0",
    "jest": "^30.2.0",
    "ts-node": "^10.9.2"
  }
}

2. create a simple jest.config.cts file:

import type { Config } from 'jest';

const config: Config = {
	rootDir: __dirname,
	testMatch: ['<rootDir>/__tests__/**/*.test.(js|ts)'],
	transform: {
		'\\.(ts|js)$': '@swc/jest',
	},
};

// eslint-disable-next-line import/no-default-export
export default config;

3. create a simple __tests__/foo.test.ts file:

describe('foo', () => {
  it('should pass this test', () => {
    expect(true).toBe(true);
  });
});

4. run npx jest

Expected behavior

No warnings

Actual behavior

A warning appears in the console:

(node:81672) Warning: Failed to load the ES module: /(...)/jest.config.cts. Make sure to set "type": "module" in the nearest package.json file or use the .mjs extension.
(Use `node --trace-warnings ...` to show where the warning was created)

This does not prevent the tests from running properly. Afaik we should not need to add "type": "module" when naming the config file using the cts extension.

Note that the warning is absent when using the mts extension.

Additional context

No response

Environment

System:
    OS: macOS 15.7.1
    CPU: (10) arm64 Apple M1 Pro
  Binaries:
    Node: 24.11.0 - /Users/jeromelouis/.nvm/versions/node/v24.11.0/bin/node
    Yarn: 1.22.22 - /Users/jeromelouis/.nvm/versions/node/v22.11.0/bin/yarn
    npm: 11.6.1 - /Users/jeromelouis/.nvm/versions/node/v24.11.0/bin/npm
    pnpm: 10.14.0 - /Users/jeromelouis/.nvm/versions/node/v22.11.0/bin/pnpm
  npmPackages:
    jest: ^30.2.0 => 30.2.0

[hainenber]

I'm afraid your .cts config didn't adhere to CommonJS spec as there are import and export syntaxes rather than require and module.export, correspondingly. That's why Node emitted out the warning.

Thankfully, Jest will try to load the file as ESM config, illustrated by below LoCs, and executed your tests :D

jest/packages/jest-util/src/requireOrImportModule.ts

Lines 64 to 69 in c12575b

	} catch (error: any) {
	if (
	error.code === 'ERR_REQUIRE_ESM' ||
	error.code === 'ERR_REQUIRE_ASYNC_MODULE'
	) {
	return importModule(filePath, applyInteropRequireDefault);

[jlowcs]

Fair point. I wasn't able to make it work with ts or mts with node 24, as it resulted in import issues, so jest was not even running. So I fugured that cts might be the solution.

That being said, one could expect that cts would be a normal TS file (using imports/exports) but compiling to CommonJS.

Using require is not really a thing for TS.

So, while that warning makes sense for cjs extensions, I'm not sure it does for cts extensions.

[hainenber]

It'd be the same for .cts as well when type stripping is enabled such as your Node version, 24 :D

[jlowcs]

When using the mts extension, I'm getting errors that seem to indicate that the jest.config.mts is not being loaded. Adding a console.log() confirms it. And running pnpm jest --showConfig 2>&1 | grep -A 10 "transform" show me a config that is not mine (probably the default jest config).

When using the ts extension, it does not work because my app is not yet set up as type:module (that would require non-trivial changes)

When using the cts extension, I guess the warning mentioned above.

What do you suggest I do?

[hainenber]

.ts file can be CommonJS, fyi.

[jlowcs]

But I don't want it to be CommonJS. I want it to be a ESM TypeScript file, like all the other TS files of my app. This was working fine with node 22.11.0, I don't understand why bumping to node 24 broke it...

[jlowcs]

My guess is that, with 22.11.0, the file was being transpiled, and, with node 24, it is being run natively by jest.

[jlowcs]

so, I got it working with a .ts with ESM, but I have to fully specify imports (with extension), which requires a TS compilerOption to be set, and is inconsistent with the rest of the codebase (which does not use extensions because we're transpiling that code to commonjs)

[hainenber]

It's sorta similar to what I have at workplace, i.e. Jest/Prettier/ESLint config at bleeding-edge .ts but TS codes containing business logic are transpiled to good ol' CJS.