First Letter to Appear Twice LeetCode solution in JavaScript

1
var repeatedCharacter = function (s) {
2
  const m = {};
3

4
  for (let i of s) {
5
    if (i in m) {
6
      m[i]++;
7
    } else {
8
      m[i] = 1;
9
    }
10

11
    if (m[i] == 2) {
12
      return i;
13
    }
14
  }
15
};