Using jest.mock's factory parameter

jest.mock takes a module factory argument - a function that returns the mock (a HOF).

To mock just the default export, return a standard HOF:

jest.mock("../moduleName", () => {
  return jest.fn(() => "default mock");
});

To mock just a named export, return an object from the function:

jest.mock("../moduleName", () => ({
  foo: jest.fn(() => "named mock"),
}));

To mock both a default and a named export, return an object with the _esModule property:

jest.mock("../moduleName", () => ({
  _esModule: true,
  foo: jest.fn(() => "named mock"),
  default: jest.fn(() => "default mock"),
}));

To only mock a particular named export:

jest.mock("../moduleName", () => {
  const actual = jest.requireActual("../moduleName");

  return {
    ...actual,
    foo: jest.fn(() => "named mock"),
  };
});

Calls to jest.mock are hoisted to the top of the code. This creates the limitation that it's not possible to first define a variable and use it inside the factory. An exception is made for variables that start with the work mock.

To specify a mock later, e.g. in a beforeAll, you can use mockImplementation on the existing mock rather than the factory parameter. This will also allow you to change the mock between tests if you wish.

[[testing]]