Sliding Window algorithm

// count number of substrings that has 3 unique characters

var countGoodSubstrings = (s) => {
  const goodSubStrings = []

  for(let i = 0; i < s.length - 2; i++) {
    const set = new Set([s[i], s[i+1], s[i+2]]) // set of unique chars
    if(set.size === 3) { // if set has 3 unique chars
      goodSubStrings.push(s[i] + s[i+1] + s[i+2]) // substring had 3 distinct chars
    }
  }

  console.log('original string:', s)
  console.log('good substrings:', goodSubStrings)
  console.log('unique good substrings:', new Set(goodSubStrings))
  
  return new Set(goodSubStrings).size
}

console.log('good substrings count:', countGoodSubstrings('xyzzaz')) // 1
console.log('======================')
console.log('good substrings count:', countGoodSubstrings('bdakjijcoeedddadfelldbda')) // 11

$ node count-good-substrings.js
original string: xyzzaz
good substrings: [ 'xyz' ]
unique good substrings: Set { 'xyz' }
good substrings count: 1
======================
original string: bdakjijcoeedddadfelldbda
good substrings: [
  'bda', 'dak', 'akj',
  'kji', 'ijc', 'jco',
  'coe', 'adf', 'dfe',
  'fel', 'ldb', 'bda'
]
unique good substrings: Set {
  'bda',
  'dak',
  'akj',
  'kji',
  'ijc',
  'jco',
  'coe',
  'adf',
  'dfe',
  'fel',
  'ldb'
}
good substrings count: 11

// count the number of chars of the longest substring of unique characters

var lengthOfLongestSubstring = function(s) {
  console.log('original:', s)

  let longestSubStrLen = 0 // hold max length substring
  let str = '' // current unique char substring
  let seen = new Set() // unique chars of current substring
  
  for(let i = 0; i < s.length; i++) {
    if(!seen.has(s[i])) { // new char is NOT in current substring
      str = str + s[i] // add new char to substring
      seen.add(s[i]) // now seen it
      longestSubStrLen = Math.max(longestSubStrLen, str.length) // recal max len
    } else { // new char is IN current substring
      str = str.substring(str.indexOf(s[i]) + 1) + s[i] // update unique substring
      seen = new Set(str.split('')) // update unique chars in substring
    }

    console.log('str:', str)
  }
  
  return longestSubStrLen
}

console.log('longest substr:', lengthOfLongestSubstring('abcabcbb')) // 3
console.log('================')
console.log('longest substr:', lengthOfLongestSubstring('aab')) // 2

$ node longest-sub-string.js
original: abcabcbb
str: a
str: ab
str: abc
str: bca
str: cab
str: abc
str: cb
str: b
longest substr: 3
================
original: aab
str: a
str: a
str: ab
longest substr: 2