Jest + React Testing Library: how do I mock out a method from some library that only affects one test block

Question

a question for using Jest to mock a method from third party library. so I have a component implemented like this

import {foo} from 'bar'
function Component() {
  // use foo somewhere in the component
}

and in the test I tried to mock out foo , so

import {foo} from 'bar'

jest.mock('bar')

test('test', () => {
 foo.mockReturnValue(...)
})

this works out fine but once I move jest.mock('bar') into every individual test block, jest would report an error saying that foo.mockReturnValue is not a function. like this

test('test', () => {
  jest.mock('bar')
  foo.mockReturnValue(...) // 👈🏻 error: `foo.mockReturnValue` is not a function
})

The problem is that, I want to mock out bar at every test level, not the global level which affects the whole test file. is there a way to do that?

Answer 1

Since you want to mock out foo at every test level, it makes more sense to keep bar mocked globally and use beforeEach() or afterEach() to reset the mocked function before or after each test runs.

import {foo} from 'bar';

jest.mock('bar');

afterEach(() => {
  foo.mockReset();
})

test('test', () => {
 foo.mockReturnValue(...);
})

You'll want to use .mockReset() to remove mocked implementations and return values from foo ; alternatively, .mockClear() can be used if the implementation and return value for foo remains the same for each test. Read more on the difference here .