Node v13 / Jest / ES6 — native support for modules without babel or esm - node.js

Question

Is it possible to test ES6 Modules with Jest without esm or babel? Since node v13 supports es6 natively have tried:
//package.json
{
…
"type": "module"
…
}
//__tests__/a.js
import Foo from '../src/Foo.js';
$ npx jest
Jest encountered an unexpected token
…
Details:
/home/node/xxx/__tests__/a.js:1
import Foo from '../src/Foo.js';
^^^^^^
SyntaxError: Cannot use import statement outside a module
When babel is added a transpiler, it works, but can es6 modules be used natively as well?

Answer 1

Yes, it is possible from jest#25.4.0. From this version, there is a native support of esm, so you will not have to transpile your code with babel anymore.
It is not documented yet, but according to this issue you have to do 3 easy steps to achieve that (At the time of writing this answer):
Make sure you don't transform away import statements by setting transform: {} in your jest config file
Run node#^12.16.0 || >=13.2.0 with --experimental-vm-modules flag
Run your test with jest-environment-node or jest-environment-jsdom-sixteen.
So your jest config file should contain at least this:
export default {
testEnvironment: 'jest-environment-node',
transform: {}
...
};
And to set --experimental-vm-modules flag, you will have to run Jest from package.json as follows (I hope this will change in the future):
"scripts": {
"test": "node --experimental-vm-modules node_modules/jest/bin/jest.js"
}
I hope, this answer was helpful to you.

Answer 2

Note that this is is still experimental, but we have documented how to test this, so there's hopefully less confusion.
https://jestjs.io/docs/en/ecmascript-modules
The steps in https://stackoverflow.com/a/61653104/1850276 are correct

Answer 3

I followed the tips provided in the accepted answer, but I added the property "type": "module" in my package.json in order to jest works properly. This is what I done:
In package.json:
"devDependencies": {
"jest": "^26.1.0",
"jest-environment-jsdom-sixteen": "^1.0.3",
"jest-environment-node": "^26.1.0"
},
"scripts": {
"test": "node --experimental-vm-modules node_modules/jest/bin/jest.js"
},
"type": "module",
"jest": {
"transform": {},
"testEnvironment": "jest-environment-jsdom-sixteen"
}

Answer 4

To run jest from "jest" extension in VSCode with "--experimental-vm-modules" flags, put this config in your global or workspaces settings.json:
"jest.nodeEnv": {
"NODE_OPTIONS": "--experimental-vm-modules"
}

Answer 5

In addition to #Radovan Kuka's answer, here's how to run Jest with ES modules, using npx:
"test:monitoring": "npx --node-arg=--experimental-vm-modules jest -f monitoring.test.js --detectOpenHandles",
The benefit is that one doesn't need to provide the absolute node_modules path.

Answer 6

Without Babel, here's a complete, minimal example that works on recent Jest versions. Run with npm test.
$ tree -I node_modules
.
├── package.json
├── src
│   └── foo.js
└── __tests__
└── foo.spec.js
package.json:
{
"type": "module",
"scripts": {
"test": "NODE_OPTIONS=--experimental-vm-modules jest"
},
"devDependencies": {
"jest": "^29.3.1"
}
}
src/foo.js:
export const bar = () => 42;
__tests__/foo.spec.js:
import {bar} from "../src/foo";
describe("foo.bar()", () => {
it("should return 42", () => {
expect(bar()).toBe(42);
});
});
The secret sauce is in the package.json: "type": "module" and NODE_OPTIONS=--experimental-vm-modules jest.
If you want to add a mock, it's a bit complicated. See this answer.