Mock External Library Functions in Jest

Suppose you import two functions from an external library in one of your js files:

import { func1, func2 } from "externalLib";

export const doSomething = () => {
  func1();
  func2();
  // ...
};

Now let's write a test for this function, and say we want to mock func1.

describe("doSomething", () => {
  jest.mock("externalLib", () => ({
    func1: jest.fn(),
  }));

  it("does something", () => {
    func1.mockImplementation(...);
    // ...
  });
});

But oh no! This test will fail - we inadvertently blew away the rest of the implementation of externalLib when we created the mock for func1. Since doSomething relies on other functions in externalLib, we get errors.

In order to preserve the rest of the implementation of externalLib and only mock func1, we need to call requireActual and spread it's return into the mock to get the implementation of the rest of the library:

describe("doSomething", () => {
  jest.mock("externalLib", () => ({
    ...jest.requireActual("externalLib"),
    func1: jest.fn(),
  }));

  it("does something", () => {
    func1.mockImplementation(...);
    // ...
  });
});

Et voilà, your test will pass ✅.