Jest Mock - inserting values into tested file

I'm having trouble inserting values into the file under test:

import device from '../device'

let execute = () => {
  if (device.isAndroid()) {
    return true
  else {
    return false
  }
}

Now for the test file:

jest.mock('../device')
import device from '../device'

describe('when the device is Android', () => {
  let device = {}

  beforeEach(() => {
    device.isAndroid = () => true
  })

  it('returns true', () => {
    let results = execute()
    expect(result).toEqual(true)
  })
})

The test fails and returns false. What am I doing wrong?