Jest Mock Factories

One of the best features of the testing framework Jest is its built-in support for mocking imports. There is a full suite of helper functions and utilities that make it very easy to completely mock an imported file with very little effort.

Jest accomplishes this by rebuilding an entire registry of every module in the dependency tree of everything you import into the test file. This happens once per test suite/file. This can be pretty expensive and slow if you have a lot of test suites or files with a lot of imports.

This module registry is being built for a Jest test suite. Jest can swap in a mocked representation of that module. The most common way to do this is with a simple line like:

jest.mock('module');

This automatically mocks this module anywhere it may be imported in the dependency tree. Meaning it replaces every function exported from it with a jest.fn().

While this is great it doesn’t actually help our slowness. We still need to build the whole module with all of its dependencies in order to identify the exported interface for the module. If we still have to add all the child dependencies of that file to the module registry then we haven’t saved ourselves anything yet.

The way to truly mock this module and prevent us from diving into it is to provide a factory function like:

jest.mock('module', () => ({
  foo: jest.fn(),
  bar: jest.fn()
}));

Providing that factory function will completely replace the module with the object you define here, and it won’t have to travel any deeper when building the module registry.