Jest: How to mock one specific method of a class

Question

Let\'s suppose I have the following class:

export default class Person {
    constructor(first, last) {
        this.first = first;
        this.last = last         


        

Answer 1

I've combined both @sesamechicken and @Billy Reilly answers to create a util function that mock (one or more) specific methods of a class, without definitely impacting the class itself.

/**
* @CrazySynthax class, a tiny bit updated to be able to easily test the mock.
*/
class Person {
    constructor(first, last) {
        this.first = first;
        this.last = last;
    }

    sayMyName() {
        return this.first + " " + this.last + this.yourGodDamnRight();
    }

    yourGodDamnRight() {
        return ", you're god damn right";
    }
}

/**
 * Return a new class, with some specific methods mocked.
 *
 * We have to create a new class in order to avoid altering the prototype of the class itself, which would
 * most likely impact other tests.
 *
 * @param Klass: The class to mock
 * @param functionNames: A string or a list of functions names to mock.
 * @returns {Class} a new class.
 */
export function mockSpecificMethods(Klass, functionNames) {
    if (!Array.isArray(functionNames))
        functionNames = [functionNames];

    class MockedKlass extends Klass {
    }

    const functionNamesLenght = functionNames.length;
    for (let index = 0; index < functionNamesLenght; ++index) {
        let name = functionNames[index];
        MockedKlass.prototype[name] = jest.fn();
    };

    return MockedKlass;
}

/**
* Making sure it works
*/
describe('Specific Mocked function', () => {
    it('mocking sayMyName', () => {
        const walter = new (mockSpecificMethods(Person, 'yourGodDamnRight'))('walter', 'white');

        walter.yourGodDamnRight.mockReturnValue(", that's correct"); // yourGodDamnRight is now a classic jest mock;

        expect(walter.sayMyName()).toBe("walter white, that's correct");
        expect(walter.yourGodDamnRight.mock.calls.length).toBe(1);

        // assert that Person is not impacted.
        const saul = new Person('saul', 'goodman');
        expect(saul.sayMyName()).toBe("saul goodman, you're god damn right");
    });
});