1
var validUtf8 = function (data) {
2
  const len = data.length;
3
  const byteBits = data.map((a) => padLeft(a.toString(2))); //convert to bit strings
4
  let bytes,
5
    i = 0;
6
  while (i < len) {
7
    bytes = 0;
8
    //count 1s in the front.
9
    while (byteBits[i].charAt(bytes) === "1") bytes++;
10

11
    //if we have only 1 byte to process expect more than 1 byte, it should fail.
12
    if (len === 1 && bytes > 0) return false;
13

14
    //UTF8 chars can't be more than 4 bytes.
15
    if (bytes > 4) return false;
16

17
    //if we're processing more than 1 byte
18
    if (bytes > 1) {
19
      //decrement for every byte that starts with "10"
20
      while (i < len - 1 && byteBits[++i].startsWith("10")) bytes--;
21

22
      //check to see if we have too many or too little of the expected bytes left
23
      if (bytes !== 1) return false;
24
    } else i++;
25
  }
26
  return true;
27
};
28

29
const padLeft = (str, size = 8, pad = "0") =>
30
  str.length >= size
31
    ? str
32
    : pad.repeat(size).substring(str.length % size) + str;