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Mocking ES6 class methods with Jest!

Jack Caldwell on September 30, 2019

Prerequisites Before you get started with this tutorial, I'm going to presume that you already have a JavaScript project that you're wor...
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Rob Cameron •

Any hints on how to restore the original functionality at the end of the test? When using this technique I'm seeing that the order the tests run in now matters—each test after this one ends up using that same mock. :(

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Jack Caldwell •

Looking at the mock function documentation - jestjs.io/docs/en/mock-function-api, you should be able to use mockFn.mockClear(), mockFn.mockReset(), or mockFn.mockRestore() depending on your needs. Hopefully this does what you need.

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Ryan •

I have tried all of these functions and still the value is either the same or undefined. Any hints?

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Matt Arau •

Have you tried afterEach(() => jest.resetAllMocks()); ?

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Amitoz Deol •

If I change the getById method to

async getById(id) {
    const url = `http://localhost:3000/api/user/{id}`;
    const response = await fetch(url);
    return await response.json();
  }

the test will still pass. This is not expected behavior I want from test

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Jack Caldwell •

I'm not quite sure what you mean. In this example the 'ProductsManager' is the class being tested. The 'ProductsClient' is being mocked so we make assumptions about its behaviour in order to test the 'ProductsManager' in isolation. It's assumed that the 'ProductsClient' would be tested separately.

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Ryan • • Edited

I'm not sure if directly modifying the prototype chain makes it impossible to reset/clear the mock.

An implementation a spying on the prototype chain seems to be working better for me i.e.

import { ProductsClient } from './ProductsClient';
import { ProductManager } from './ProductManager';

it('should return the product', async () => {
  const expectedProduct = {
    id: 1,
    name: 'football',
  };

   jest.spyOn(ProductsClient.prototype, 'getById')
     .mockReturnValue(expectedProduct);

  const productManager = new ProductManager();
  const result = await productManager.getProductToManage(1); 

  expect(result.name).toBe('football'); // It passes!
});
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Moussa •

i get ReferenceError: regeneratorRuntime is not defined

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Maxime Lafarie • • Edited

Very clever to mock the method through the prototype. I was struggling for hours, thank you!

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jab3z •

Is there a reason why using

mockGetById.mockReturnValue(Promise.resolve(expectedProduct))

and not .resolves?

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Carlos •

thank you! I'm wondering why it's not written on the jest docs that we have to override the method through prototype.