I have a helper function helper and a setup function setUp in my tests. The helper works fine, but the setup function returns an error, because it is not found. Why? Am I missing something obvious?

function helper(arg){
  return arg+1
}

function setUpSpy(){
  let spy;

  beforeEach(()=>{
    spy = sinon.spy()
    document.addEventListener('message', spy);
  })

  afterEach(()=>{
    document.removeEventListener('message', spy);
  })
  return spy
}


describe('does a thing', function(){
  let spy = setUpSpy() // FAILS

  it('via an event', async function(){
    const app = new window.App()
    await app.init()
    console.log(helper(1)) // WORKS

    assert.equal(spy.callCount,1)
  });
});

Error returned:

 TypeError: Cannot read properties of undefined (reading 'callCount')