Step-by-step guide for resetting imported module between tests

Imagine I have a component that must be initialized once at the beginning of the application to pass on configuration. The module structure would look something like this:

MyComponent.js

let isInitialized;

const myComponent = {
    init: function() {
        isInitialized = true;
    },
    do: function() {
        if (!isInitialized)
            throw "error"
        //DO THINGS
    }
}

export default myComponent;

I want to write unit tests for it using jest. The test file would look something like this:

MyComponent.test.js

import myComponent from './MyComponent'

describe('MyComponent', () => {
    describe('init', () => {
        it('should not throw an exception when called', () => {
            expect(() => myComponent.init()).not.toThrow();
        });
    })
    describe('do', () => {
        it('should throw an exception when not initialized', () => {
            expect(() => myComponent.do()).toThrow();
        });
    })
})

However, when running the test, the second test fails because the module is already initialized and the exception is not thrown. I attempted using jest.resetModules() in beforeEach, but that did not resolve the issue.

Is there a solution to this problem, possibly by changing the module pattern or test case?

Answer №1

You need to either re-import or re-require your module. Visit the documentation or refer to this issue for further details:

https://github.com/facebook/jest/issues/3236

https://jestjs.io/docs/jest-object#jestresetmodules

describe('MyModule', () => {
    beforeEach(() => {
        jest.resetModules()
    });

    describe('init', () => {
        const myModule = require('./MyModule');

        it('does not throw an exception when called', () => {
            expect(() => myModule.init()).not.toThrow();
        });
    })
    describe('do', () => {
        const myModule = require('./MyModule');

        it('throws an error when not initialized', () => {
            expect(() => myModule.do()).toThrow();
        });
    })
})

Answer №2

When working with the `import` statement instead of `require`, you may encounter an error like this:

SyntaxError: /path/to/test/file.js: 'import' and 'export' may only appear at the top level

To address this issue, consider using the babel-plugin-dynamic-import-node plugin to reset the modules. Here's an example of how this can be implemented:

describe('MyTests', () => {
  let MyModule;

  beforeEach(() => {
    return import('../module/path').then(module => {
      MyModule = module;
      jest.resetModules();
    });
  });

  test('should test my module', () => {
    expect(MyModule.aMethod).not.toBeUndefined();
  });
});

Source: https://github.com/facebook/jest/issues/3236#issuecomment-698271251

Answer №3

In my situation, simply re-requiring the module was not effective. After consulting the jest documentation, I came across this. The solution presented involved using jest.resetModules(). Essentially, this function just clears the cache so that when you re-require a module, it is loaded from scratch. Therefore, whenever you need to re-require something, ensure you utilize this method to guarantee a fresh loading process.

Answer №4

After encountering a challenge with maintaining canonical top-of-the-file imports, I devised a solution that proved to be effective for me. This involved the addition of a specific function

export function resetIsInitiazliedForTests() {
  isInitiazlied = false
}

to the file MyModule.js. Subsequently, I made use of this function within the beforeEach block in the test file MyModule.test.js.

Answer №5

When it comes to working with Jest across different Node platforms, resetting the process.env may not always yield the desired results. Through experimentation and trial, I discovered a simple solution to address this issue by utilizing a regular Jest mock and incorporating a wrapper function to retrieve process.env. This approach can help prevent unexpected behaviors that may arise across various platforms.

If you are new to Jest, here is a comprehensive solution for you. Start by creating a JS file named nodeEnvironment.js and defining the following function:

const getNodeEnvironment = () => {
  return process.env;
};

export default getNodeEnvironment;

Next, import the getNodeEnvironment function as you normally would:

import getNodeEnvironment from './nodeEnvironment';

const myMainFunction= () => {
     const env = getNodeEnvironment();
     const myVarVal = env.myVar    
 }

Finally, implement the Jest mock. Remember to set the esModule flag to true if you want your function to be the default export, and then add 'return default: mockvalue' in this manner:

jest.mock('./nodeEnvironment', () => ({
  __esModule: true,
  default: () => {
    // Define all necessary variables for your tests here
    const env = { myVar: "123" }; 
    return env;
  },
}));

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