function isDefined<T>(argument: T | undefined): argument is T {
  return typeof argument !== 'undefined'
}

const items = [1, 2, undefined]

items.filter(isDefined).map((number) => {
  // number is now type guarded and undefined is eliminated
  console.log(number)
})

items.filter((num) => isDefined(num)).map((number) => {
  // however, this way it does not work, and "number" can still be undefined
  console.log(number)
})