Specify code to run before any Jest setup happens

Question

The tl;dr is:

1) How can I have Jest use the native require function to load all modules in my tests anywhere.

2) Where / how would I go about modif

Answer 1

I tried using node -r @std/esm run.js where run.js is just a script that calls jest, but it does not work and crashes here : https://github.com/facebook/jest/blob/master/packages/jest-runtime/src/script_transformer.js#L305.

From what I understand from this line means that it is not possible because jest compiles the module using the native vm module. The above lines (290):

  if (willTransform) {
    const transformedSource = this.transformSource(
    filename,
    content,
    instrument,
    !!(options && options.mapCoverage));


    wrappedCode = wrap(transformedSource.code);
    sourceMapPath = transformedSource.sourceMapPath;
  } else {

is the code called when you are specifying transforms in your jest config.

Conclusion : until esm are supported ( and they will be under the .mjs extension ) you cannot import es modules in jest without specifying a transform. You could try to monkey patch vm but I would really advise against this option.

Specifying a jest transform is really not that hard, and for es modules it's really as simple as using babel-jest with the right babel config :

Below a package.json with minimal settings

{
    "dependencies": {
        "babel-jest": "^21.2.0",
        "babel-plugin-transform-es2015-modules-commonjs": "^6.26.0",
        "jest": "^21.2.1"
    },
    "jest": {
        "testMatch": [
            "/src/**/__tests__/**/*.js?(x)",
            "/src/**/?(*.)(spec|test).js?(x)"
        ],
        "transform": {
            "^.+\\.(js|jsx)$": "/node_modules/babel-jest"
        },
        "testEnvironment": "node",
        "testURL": "http://localhost",
        "moduleFileExtensions": [
            "js",
            "json"
        ]
    },
    "babel": {
        "plugins": ["babel-plugin-transform-es2015-modules-commonjs"]
    }
}