Sometimes, we want to mock one specific method of a class with Jest and JavaScript.

In this article, we’ll look at how to mock one specific method of a class with Jest and JavaScript.

How to mock one specific method of a class with Jest and JavaScript?

To mock one specific method of a class with Jest and JavaScript, we can use the jest.spyOn method.

For instance, we write

import Person from "./Person";

test("Modify only instance", () => {
  let person = new Person("foo", "bar");
  let spy = jest.spyOn(person, "sayMyName").mockImplementation(() => "Hello");

  expect(person.sayMyName()).toBe("Hello");
  expect(person.bla()).toBe("bla");
  spy.mockRestore();
});

to mock the person.sayMyName method by calling spyOn and mockImplementation in the test.

The mockImplementation callback has the faked implementation of person.sayMyName.

Likewise, we can mock the same method for all tests by writing

import Person from "./Person";

beforeAll(() => {
  jest.spyOn(Person.prototype, "sayMyName").mockImplementation(() => "Hello");
});

afterAll(() => {
  jest.restoreAllMocks();
});

test("Modify class", () => {
  let person = new Person("foo", "bar");
  expect(person.sayMyName()).toBe("Hello");
});

We call jest.spyOn with Person.prototype and 'sayMyName and mockImplementation to do the mocking in the beforeAll callback.

The beforeAll callback runs before any tests are run.

Then we call restoreAllMocks to restore all mocks after all tests are run.

Conclusion

To mock one specific method of a class with Jest and JavaScript, we can use the jest.spyOn method.