How to use ESM tests with jest?

Question

My setup

• Node v14.16.0
• I have one test file that contains a simple test written with ESM modules (import).
• In package.json I have a script (following Jest documentation adapted to Windows): "test": "set \"NODE_OPTIONS=--experimental-vm-modules\" && npx jest"
• My tests are run w/ npm run test (the script above)

My goal: Run the ESM tests using jest.

My attempts:

(1) CommonJS App w/ .js test

• package.json contains "type": "commonjs"
• Test file named test.js
• Error: SyntaxError: Cannot use import statement outside a module (This is expected because the test is being run as a commonjs file.)

(2) CommonJS App w/ .mjs test

• package.json contains "type": "commonjs"
• Test file named test.mjs
• Error: No tests found (Jest isn't finding the file?)

(3) ESM App w/ .js or .mjs

• package.json now has "type": "module"
• Test file named test.mjs or test.js
• Error: ReferenceError: module is not defined at ...jest.config.js:6:1

(4) CommonJS App and CommonJS test

• package.json contains "type": "commonjs"
• Test file named test.js but using the commonjs require syntax (and without the ESM modules I'd like to include); Note that this is not what I want, but included just to show there's nothing wrong with the test itself.
• Test runs fine and passes

Answer 1

Here are the steps I took to run Jest with a test using ESM. The source files under test were also written using ESM.

1. Set my node version to 14.16.0
2. npm i jest -D installs Jest as a dev dependency
3. Add "type": "module" to package.json
4. Update scripts in package.json to include "test": "node --experimental-vm-modules ./node_modules/.bin/jest"
5. Create a jest.config.js file with the following content: export default { transform: {} }
6. Have at least one test that could be found using the default testMatch (mine was inside __tests__ dir)
7. npm test

There is a more complete example in the webpack-strip-debug-loader repository on GitHub.

Answer 2

For me to work it was only necessarry to set script "test" as follows:

"test": "cross-env NODE_OPTIONS=--experimental-vm-modules npx jest"

and yeah, don't forget -> npm i cross-env jest

Answer 3

After a lot of fiddling, this is my successful setup (the parts that matter):

• package.json has

  "type": "commonjs",
  "script": {
    "test": "NODE_OPTIONS=--experimental-vm-modules jest"
  }

• jest.config.mjs (yes, esModule)

export default {
  moduleFileExtensions: [
    "mjs",
    // must include "js" to pass validation https://github.com/facebook/jest/issues/12116
    "js",
  ],
  testRegex: `test\.mjs$`,
};

That having to include "js" is a historic relic from the times modules where obviously .js files. Follow https://github.com/facebook/jest/issues/12116 for update.

Answer 4

Here's the minimum you need. Props to @Luke for the comment above with the testMatch answer I needed.

1. package.json - Make sure "type": "module" is in there

2. jest.config.js - Add this glob to your testMatch array: "**/?(*.)+(spec|test).(m)js"

3. Make sure your test file(s) end in .spec.mjs or .test.mjs to match the glob

4. Set your npm test script to node --experimental-vm-modules ./node_modules/.bin/jest

That's it. You do not need the transform setting in the jest config as the documentation suggests.

Answer 5

in my case, leaving

• "type": "module" in package.json

and renaming

• jest.config.js to jest.config.cjs
• babel.config.js to babel.config.cjs

worked

Answer 6

I think your issue lies in this line and how you're retrieving the tag info from the CSV object from your foreach loop:

$scriptBlock = [scriptblock]::Create('@{$info.Tags}')

In the previous line in your script, to define the $tags variable, you retrieve the value of the name of the VM in the CSV object using ($info.psobject.properties.value[0]). I would attempt to use that same format in the next line, but modifying it to retrieve the tag part of the CSV object using the index of 1:

So instead of:

$scriptBlock = [scriptblock]::Create('@{$info.Tags}')

use:

$scriptBlock = [scriptblock]::Create('@{$info.psobject.properties.value[1]}')