3 Ways to Mock Named Exports in Jest

In order to mock named exports in Jest, you need to import all named exports from a file with an * and assign jest.fn to the method that needs to be mocked.

import * as utils from 'utils'

utils.sum = jest.fn()

We can use an alias for the import so that we can access all named exports within an object. Now we can simply reassign one of the methods of utils to a mock function using jest.fn. We can also mock the implementation using mockImplementation:

import * as utils from 'utils'

const original = utils.sum

utils.sum = jest.fn().mockImplementation(() => {
    // Mock implementation goes here
})

// Restore mock
utils.sum = jest.fn().mockImplementation(original)

mockImplementation expects a callback function to be passed that describes the implementation. In case you need to restore the mock later on, we can save the original function into a variable, and call mockImplementation again with the original function.

You may also simply want to mock only the return value instead of the whole implementation. In order to mock a return value in Jest, we can use mockReturnValue.

import * as utils from 'utils'

utils.sum = jest.fn().mockReturnValue(2)