How to mock modules with jest

This repository is showing examples how to mock a module using jest. It is about modules which were installed into node_modules via package manager. There are few javascript examples but also the more challenging typescript examples. You can find the examples within a list of tests in directories tests_javascript and tests_typescript.

I am using the cross-fetch, a library similar to node-fetch, but with better typescript support. The library has a default_export as well as named exports.

Why this?

When reading the documentation on https://jestjs.io I found it too difficult to put all pieces together ( As of January 2023). Thefore I created these examples with the focus of simplicity. This is one way to do it. Maybe there are others.

Typescript examples

File	Description
01_no_mock_implementation.ts	Shows how jest.mock('<module_name'>) immediately mocks any exported object
02_mock_default_export.ts	All exported objects from cross-fetch are mocked. An implementation is only defined for the default exported fetch function
03_mock_named_export.ts	All exported objects are mocked. An implementation is only defined for the named export of Headers class
04_mock_partials.ts	All exported objects are mocked. The original implementation is restored for the Response class. A custom implementation has been defined for the default export fetch
05_mostly_original_implementation.ts	All exported objects have their original implementation, except the default export fetch

Javascript examples

File	Description
02_mock_default_export.js	See above
05_mostly_original_implementation.js	See above

Usage

npm install
npm run test

References

• https://jestjs.io
• https://www.typescriptlang.org/docs/