Gist exemplifying usage of Jest module mocks, with Flow.

i18n.js (mocked)

// modules/__mocks__/i18n.js

const i18n = {
  t: name => `Bye ${name}!`
}

export default i18n

i18n.js (original)

// ./modules/i18n.js

const i18n = {
  t: name => `Hello ${name}!`
}

export default i18n

utils.js

// ./utils.js

import i18n from './modules/i18n'

export const functionThatUsesT = name => `${i18n.t(name)}`

utils.spec.js

// ./utils.spec.js

import { functionThatUsesT } from './utils'

jest.mock('./modules/i18n')

describe('functionThatUsesT', () => {

  it('should mock i18n and say bye instead of hello', () => {
    expect(functionThatUsesT('Spiderman')).toEqual('Bye Spiderman!')
  })

}