How to use mockFetchData method in Jest

Please, add the following to your .eslintrc file:

{
  "overrides": [
    {
      "files": [
        "**/*.spec.js",
        "**/*.spec.jsx"
      ],
      "env": {
        "jest": true
      }
    }
  ]
}

1. I think it depends on what it is you are testing. If the process you are testing with jest requires 5Gb or RAM then I could see it happening. It is probable that it is the process that you are testing causing the memory usage.

2. From my experience, no, Lodash and react-dom are smallish libraries that do not require a lot of RAM (react-dom especially). Lodash's RAM usage depends on its task once more, if you are using it to alter or compute large arrays then the RAM usage will go up.

3. Yes, having wasted roughly 1 month looking for a memory leak in my Electron application, I found it with React-devTools. It seemed like the devtools would keep track (in-memory logs) of all the updates taking place in react under the hood and add to the total ram location of the app. It could be possible that jest is loading up react-devTools somehow? Seems a bit odd to me but anything is possible.

If both lodash and react-dom are using RAM together, it sounds like it might be a high amount of rerenders/ routing taking place, especially if you are using lodash to compute strings or arrays based off of the routes. If you are not using class react components (thus using functional components) I would recommend switching and using componentDidMount() and componentWillUnmount() to reduce the probability of memory leaks. You can check out the documentation here.

Or better yet, add

yarn add --dev identity-obj-proxy

And add the following to jest.config.json

{
  "jest":{
     "moduleNameMapper": {
      "\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$": "<rootDir>/__mocks__/fileMock.js",
      "\\.(css|less|scss|sass)$": "identity-obj-proxy"
    },
  }
}

Add to workspace settings:

{
    "eslint.workingDirectories": [
        "back",
        "front"
    ]
}

and potentially:

{
    "eslint.workingDirectories": [
        { 
            "directory": "front", 
            "changeProcessCWD": true
        }
    ]
}

The problem is with where vscode/eslint looks for dependencies.

Just restart your server:

npm run serve

You can set console.log to by a spy like this:

global.console = {
  warn: jest.fn(),
  log: jest.fn()
}

// run your code

expect(global.console.log).toHaveBeenCalledWith('test')

As your test file runs in a separate thread you don't need to reset console to the original methods

The timeout you specify here needs to be shorter than the default timeout.

The default timeout is 5000 and the framework by default is jasmine in case of jest. You can specify the timeout inside the test by adding

jest.setTimeout(30000);

But this would be specific to the test. Or you can set up the configuration file for the framework.

Configuring Jest

// jest.config.js
module.exports = {
  // setupTestFrameworkScriptFile has been deprecated in
  // favor of setupFilesAfterEnv in jest 24
  setupFilesAfterEnv: ['./jest.setup.js']
}

// jest.setup.js
jest.setTimeout(30000)

See also these threads:

setTimeout per test #5055

Make jasmine.DEFAULT_TIMEOUT_INTERVAL configurable #652

P.S.: The misspelling setupFilesAfterEnv (i.e. setupFileAfterEnv) will also throw the same error.

Hm. I've found a solution.

See. A change in my file with the target function.

const myClass = require('./myClass');
// const instance = new myClass(); <== Not here...

module.exports.functionToTest = async function () {
    const instance = new myClass(); // <== ...but there.

    // Some stuff...

    const value = await instance.myMethod();

    // Some stuff that define a result variable (partially with value).

    return result;
} 

And my spec file :

const myFile = require('./myFile');

// I specify to Jest that I'll mock a file
jest.mock('./myClass');
const myClass = require('./myClass');

// I prepare the mock function. In that case a promise wich resolve 'false'
const mMock = jest.fn().mockResolvedValue(false);

// I mock the method 'myMethod' in 'myClass'
myClass.mockImplementation(() => {
    return {
        myMethod: mMock
    };
});


// Then, I just take the test
describe('My test', async () => {
    it('should mock myClass.myMethod in order to return false', () => {
        const result = await myFile.functionToTest();

        expect(result).toBeFalsy();
    }
}

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.