Does Cypress support runtime mocking of internal variables like Jest does?

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I'm looking to change the value of an internal variable within a function or React component at runtime—similar to how Jest allows module mocking using jest.mock. For instance, in Jest I can do the following:

JAVASCRIPT
const getActionTypeName = jest.fn(() => {});

jest.mock('./pathToModule', () => {
  return {
    __esModule: true,
    getActionTypeName: () => getActionTypeName(),
  }
})

describe('functionBeingTested', () => {
  it('should do something', () => {
    getActionTypeName.mockReturnValue('RUNTIME_RESET_TYPE');
    // test code...
    // assertions...
  });
});

Can Cypress achieve something similar at runtime, or does it require another approach? Additionally, are there alternative strategies for mocking internal module variables or functions when using Cypress for testing?

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

Cypress doesn't provide a built‑in equivalent to Jest’s module mocking mechanism that lets you swap out internal variable values or module dependencies at runtime for unit tests. Cypress is primarily focused on end‑to‑end/integration testing in a browser, and its architecture doesn't lend itself to the same level of runtime module override that Jest’s Node‑based environment supports.

Key Points

Different Focus:
Cypress is designed for testing the application as it runs in a browser (e.g., real user interactions), whereas Jest focuses on unit testing and allows you to intercept module imports via its mocking API.
Runtime Module Overrides:
In Jest you can easily override a module by using jest.mock and then modify internal functions or variables via jest.fn. In Cypress, once the code is bundled and served to the browser, it’s not trivial to intercept or replace internal variables of a module in the same manner.
Alternatives in Cypress:
- Dependency Injection: If you need to swap behaviors during an integration test, consider designing your application to support dependency injection. This allows you to inject mocks or stubs without needing runtime module replacement.
- Stubbing Browser APIs/Functions: Cypress has cy.stub which can replace functions on objects that are exposed in the browser (for example, window properties or API endpoints) but this won’t work for internal variables or closed-over module-scoped functions.
- E2E Workflow Adjustments: For scenarios needing module-level mocks, the control flow might need to remain in unit test environments like Jest where module mocking is better supported rather than trying to retrofit it into Cypress.

Conclusion

While you can perform some stubbing in Cypress, it doesn’t offer the same internal module mocking capabilities as Jest because they serve different purposes. For unit tests that require internal function or variable overrides, Jest remains the better tool, whereas Cypress is best used for high‑level integration or end‑to‑end scenarios.

If you need to evaluate or verify behaviors that depend on internal values, consider testing those parts with Jest or redesigning your component to allow dependency injection, which can then be stubbed or spied upon in Cypress.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by StarlitCosmonaut037 • 1 month ago

One way that modules imported by the app can be mocked is for the app to attach the import to window, then the test can grab it and apply cy.stub() in an onLoad event, or as a separate command based on cy.window().

Roughly the pattern is as follows. Somewhere in the React app code

JAVASCRIPT
import myModule from './pathToModule';

...

if (window.Cypress) {         // If Cypress loads the app, window will have
                              // a reference to the Cypress object
  window.myModule = myModule; // so the app knows to expose the imported instance
}                             // of myModule

Then in the test, where you want to apply mock, use cy.stub() which is a command wrapper for a Sinon stub (see Cypress.sinon and Stubs, Spies, and Clocks).

JAVASCRIPT
cy.window().then(win => {
  if (win.myModule) {
    cy.stub(win.myModule, 'getActionTypeName')
      .returns('RUNTIME_RESET_TYPE')
      // or .callsFake(() => 'RUNTIME_RESET_TYPE')
      .as('getActionTypeName');                     // set alias for checking 
                                                    // that the call was made
  }
})

... // some actions to invoke the stubbed method

cy.get('@getActionTypeName').should('have.been.called')

See Sinon-Chai Assertions for examples of various other assertion expressions.

Gleb Bahmutov gives an example here Stub Objects By Passing Them Via Window Property

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Does Cypress support runtime mocking of internal variables like Jest does?

2 Answers

Key Points

Conclusion

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