My codes are：

utils.js:

exports.a = function a(){
 b()
}
function b() {}
exports.b = function b(){
}

utils.test.js:

var utils = require('./utils')
test('b called by a', () => {
  u.b = jest.fn()

  utils.a()
  expect(utils.b).toHaveBeenCalled()
})

When run unit test,it failed with:

Expected number of calls: >= 1
Received number of calls:    0

How to test it in a correct way?