1
const getCharIdx = (c) => c.charCodeAt(0) - "a".charCodeAt(0);
2
const isEqual = (a, b) => a.every((v, i) => v == b[i]);
3

4
var checkInclusion = function (s1, s2) {
5
  const occS1 = new Array(26).fill(0);
6
  const occS2 = new Array(26).fill(0);
7

8
  const s1Len = s1.length,
9
    s2Len = s2.length;
10

11
  if (s1Len > s2Len) return false;
12

13
  let l = 0,
14
    r = 0;
15
  for (; r < s1Len; r++) {
16
    occS1[getCharIdx(s1[r])]++;
17
    occS2[getCharIdx(s2[r])]++;
18
  }
19

20
  if (isEqual(occS1, occS2)) {
21
    return true;
22
  }
23

24
  for (; r < s2Len; r++) {
25
    occS2[getCharIdx(s2[r])]++;
26
    occS2[getCharIdx(s2[l++])]--;
27

28
    if (isEqual(occS1, occS2)) return true;
29
  }
30

31
  return false;
32
};