Jest 'TypeError: is not a function' in jest.mock

Question

I'm writing a Jest mock, but I seem to have a problem when defining a mocked function outside of the mock itself.

I have a class:

myClass.js

class MyClass {
  constructor(name) {
    this.name = name;
  }

  methodOne(val) {
    return val + 1;
  }

  methodTwo() {
    return 2;
  }
}

export default MyClass;

And a file using it:

testSubject.js

import MyClass from './myClass';

const classInstance = new MyClass('Fido');

const testSubject = () => classInstance.methodOne(1) + classInstance.name;

export default testSubject;

And the test:

testSubject.test.js

import testSubject from './testSubject';

const mockFunction = jest.fn(() => 2)

jest.mock('./myClass', () => () => ({
    name: 'Name',
    methodOne: mockFunction,
    methodTwo: jest.fn(),
}))


describe('MyClass tests', () => {
    it('test one', () => {
        const result = testSubject()

        expect(result).toEqual('2Name')
    })
})

However, I get the following error:

TypeError: classInstance.methodOne is not a function

If I instead write:

...
methodOne: jest.fn(() => 2)

Then the test passes no problem.

Is there a way of defining this outside of the mock itself?

Answer 1

In my case, I had to mock a Node.js module. I'm using React and Redux in ES6, with Jest and Enzyme for unit tests.

In the file I'm using, and writing a test for, I'm importing the node modules as default:

import nodeModulePackage from 'nodeModulePackage';

So I needed to mock it as a default since I kept getting the error (0, _blah.default) is not a function..

My solution was to do:

jest.mock('nodeModulePackage', () => jest.fn(() => {}));

In my case, I just needed to override the function and make it return an empty object.

If you need to call a function on that node module, you'll do the following:

jest.mock('nodeModulePackage', () => ({ doSomething: jest.fn(() => 'foo') }));

Answer 2

I figured this out. It is to do with hoisting, see: Jest mocking reference error

The reason it had worked in a previous test, where I had done it, was because the testSubject was itself a class. This meant that when the testSubject was instantiated, it was after the variable declaration in the test file, so the mock had access to use it.

So in the above case it was never going to work.

Answer 3

Defining mockOne as an unassigned let and then initialising the variable inside the mocking function worked for me:

let mockFunction

jest.mock('./myClass', () => () => {
    mockFunction = jest.fn(() => 2)
    return {
        name: 'Name',
        methodOne: mockFunction,
        methodTwo: jest.fn(),
    }
}))