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 modifying (ie replacing with the esm loader) https://github.com/standard-things/esm the require function in one place, before any tests run, so all tests will use the modified require.

---

I'd like to use the esm-loader with my Jest test files. In order to do so, I need to patch the require function globally, before any test code runs, with something like

require = require("@std/esm")(module, { esm: "js", cjs: true });

How do I tell Jest to execute that code before anything else is touched or requested?

I tried pointing both setupTestFrameworkScriptFile and an setupFiles array entry to a file with that in it, but neither worked (though I did confirm that both ran).

Alternatively, I'm firing off these tests with an npm script

"scripts": {
  "test": "jest"
}

Is there some CLI magic whereby I can just load a module and then run jest?

---

Edit - the testEnvironment and resolver options make me wonder if this is ever even using the actual Node require function to load modules, or instead using its own module loader. If so I wonder if this is even possible.

Answer 1

So this one was a bit tough to get working. The solution is quite simple but it took me a while to get it working. The problem is that whenever you use any module in jest

• Setup Files
• Setup Framework Files
• Test Files
• Module files

They are all loaded in below way

({"Object.":function(module,exports,require,__dirname,__filename,global,jest){/*Module code inside*/ }});

If you have a look at node_modules/jest-runtime/build/index.js:495:510

const dirname = (_path || _load_path()).default.dirname(filename); localModule.children = []; localModule.parent = mockParentModule; localModule.paths = this._resolver.getModulePaths(dirname); localModule.require = this._createRequireImplementation(filename, options);  const transformedFile = this._scriptTransformer.transform( filename, {   collectCoverage: this._coverageOptions.collectCoverage,   collectCoverageFrom: this._coverageOptions.collectCoverageFrom,   collectCoverageOnlyFrom: this._coverageOptions.collectCoverageOnlyFrom,   isInternalModule,   mapCoverage: this._coverageOptions.mapCoverage },  this._cacheFS[filename]); 

this._createRequireImplementation(filename, options); gives every module a custom require object. So you as such don't get the native require function at all, anywhere. Once jest has started every module loaded from then on will have jest's custom require function.

When we load a module, the requireModule methods from the jest-runtime gets called. Below is an excerpt from the same

  moduleRegistry[modulePath] = localModule;   if ((_path || _load_path()).default.extname(modulePath) === '.json') {     localModule.exports = this._environment.global.JSON.parse(     (0, (_stripBom || _load_stripBom()).default)((_gracefulFs || _load_gracefulFs()).default.readFileSync(modulePath, 'utf8')));    } else if ((_path || _load_path()).default.extname(modulePath) === '.node') {     // $FlowFixMe     localModule.exports = require(modulePath);   } else {     this._execModule(localModule, options);   } 

As you can see if the extension of the file is .node it loads the module directly, else it calls the _execModule. This function is the same code that I posted earlier which does the code transformation

const isInternalModule = !!(options && options.isInternalModule); const filename = localModule.filename; const lastExecutingModulePath = this._currentlyExecutingModulePath; this._currentlyExecutingModulePath = filename; const origCurrExecutingManualMock = this._isCurrentlyExecutingManualMock; this._isCurrentlyExecutingManualMock = filename;  const dirname = (_path || _load_path()).default.dirname(filename); localModule.children = []; localModule.parent = mockParentModule; localModule.paths = this._resolver.getModulePaths(dirname); localModule.require = this._createRequireImplementation(filename, options); 

Now when we want to modify require function for our test, we need _execModule to export our code directly. So the code should be similar to loading of a .node modules

  } else if ((_path || _load_path()).default.extname(modulePath) === '.mjs') {     // $FlowFixMe     require = require("@std/esm")(localModule);     localModule.exports = require(modulePath);   } else { 

But doing that would mean patching the code, which we want to avoid. So what we do instead is avoid using the jest command directly, and create our own jestload.js and running that. The code for loading jest is simple

#!/usr/bin/env node /**  * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.  *  * This source code is licensed under the MIT license found in the  * LICENSE file in the root directory of this source tree.  */  cli = require('jest/bin/jest'); 

Now we want to modify the _execModule before the cli loads. So we add below code

const jestRuntime = require("jest-runtime"); oldexecModule = jestRuntime.prototype._execModule;  jestRuntime.prototype._execModule = function (localModule, options) {     if (localModule.id.indexOf(".mjs") > 0) {         localModule.exports = require("@std/esm")(localModule)(localModule.id);         return localModule;     }     return oldexecModule.apply(this, [localModule, options]); };  cli = require('jest/bin/jest'); 

Now time for a test

//__test__/sum.test.js sum = require('../sum.mjs').sum;   test('adds 1 + 2 to equal 3', () => {   expect(sum(1, 2)).toBe(3); });   test('adds 2 + 3 to equal 5', () => {   expect(sum(3, 2)).toBe(5); }); 

And a sum.mjs file

export function sum (x, y) { return x + y } 

Now we run the test

The solution is available on below repo

https://github.com/tarunlalwani/jest-overriding-require-function-stackoverflow

You can clone and test the solution by running npm test.

Answer 2

setupFiles worked for me. Add this in package.json:

"jest": {
    "setupFiles": ["./my_file.js"]
  },

https://jestjs.io/docs/en/configuration.html#setupfiles-array