Does Array.prototype.sort(cmp) call cmp() for undefined elements?

No.

I was reading my daily set of random reference pages and found out that poor undefined always goes last regardless of your comparison function. An interesting find.

  const arr = ['saab', 'toyota', undefined, 'volvo']
  arr.sort((a, b) => {
      console.log('cmp', a, 'vs', b)
      if (String(a) < String(b)) return -1
      if (String(a) > String(b)) return  1
      return 0
  })
  console.log(arr)

  cmp toyota vs saab
  cmp volvo vs toyota
  [ 'saab', 'toyota', 'volvo', undefined ]

If compareFn is supplied, all non-undefined array elements are sorted according to the return value of the compare function (all undefined elements are sorted to the end of the array, with no call to compareFn).