How to mock es6 class using Jest

Question

I am attempting to mock a class Mailer using jest and I can\'t figure out how to do it. The docs don\'t give many examples of how this works. The process is the

Answer 1

Just for Googlers and future visitors, here's how I've setup jest mocking for ES6 classes. I also have a working example at github, with babel-jest for transpiling the ES module syntax so that jest can mock them properly.

__mocks__/MockedClass.js

const stub = {
  someMethod: jest.fn(),
  someAttribute: true
}

module.exports = () => stub;

Your code can call this with new, and in your tests you can call the function and overwrite any default implementation.

example.spec.js

const mockedClass = require("path/to/MockedClass")(); 
const AnotherClass = require("path/to/AnotherClass");
let anotherClass;

jest.mock("path/to/MockedClass");

describe("AnotherClass", () => {
  beforeEach(() => {
    mockedClass.someMethod.mockImplementation(() => {
      return { "foo": "bar" };
    });

    anotherClass = new AnotherClass();
  });

  describe("on init", () => {
    beforeEach(() => { 
      anotherClass.init(); 
    });

    it("uses a mock", () => {
      expect(mockedClass.someMethod.toHaveBeenCalled();
      expect(anotherClass.settings)
        .toEqual(expect.objectContaining({ "foo": "bar" }));
    });
  });

});