How to unit test a promise deliberately designed to side-effect?

test.js

// demo.js
function f() {
  return Promise.resolve(true)
}

function g() {
  Promise.resolve(true)
}

module.exports = { f, g }

// demo.test.js
const demo = require('./demo')

describe('f()', () => {
  it('should resolve to true', () => {
    return demo.f().then(result => {
      expect(result).toBe(true)
    })
  })
})

describe('g()', () => {
  it.skip('should resolve to true', () => {
    // ???
  })
})