function(state) { return state.createEntity.cancelItems.map((i) => i.bookingItemId }

import unittest
class TestFunction(unittest.TestCase):
  def test_function(self):
    state = {
      createEntity: {
        cancelItems: [
          {
            bookingItemId: 1,
          },
          {
            bookingItemId: 2,
          },
        ],
      },
    }

    self.assertEqual(function(state), [1, 2])