Jest: How to mock one specific method of a class

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Using jest.spyOn() is the proper Jest way of mocking a single method and leaving the rest be. Actually there are two slightly different approaches to this.

1. Modify the method only in a single object

import Person from "./Person";

test('Modify only instance', () => {
    let person = new Person('Lorem', 'Ipsum');
    let spy = jest.spyOn(person, 'sayMyName').mockImplementation(() => 'Hello');

    expect(person.sayMyName()).toBe("Hello");
    expect(person.bla()).toBe("bla");

    // unnecessary in this case, putting it here just to illustrate how to "unmock" a method
    spy.mockRestore();
});

2. Modify the class itself, so that all the instances are affected

import Person from "./Person";

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

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

test('Modify class', () => {
    let person = new Person('Lorem', 'Ipsum');
    expect(person.sayMyName()).toBe("Hello");
    expect(person.bla()).toBe("bla");
});

And for the sake of completeness, this is how you’d mock a static method:

jest.spyOn(Person, 'myStaticMethod').mockImplementation(() => 'blah');

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