Longest string without repeating characters in javaascript

 * @param {string} s * @return {number} */ var lengthOfLongestSubstring = function(s) { let map = new Map(); if (!s.length) { return null } for (let i = 0; i < s.length; i++) { let count = 0; let checkString = s.substring(0, i + 1) let start = 0; while (s.substring(start, s.length - 1).indexOf(checkString) > -1) { count++; if (map.has(checkString)) { let existedCount = map.get(checkString); map.set(checkString, existedCount + 1) existedCount = 0; } else { map.set(checkString, count) } start++; } } if (map.size) { let keys = Array.from(map.keys()); return keys.reduce((a, b) => map.get(a) > map.get(b) ? map.get(a) : map.get(b)) } }; //console.log("Higest", lengthOfLongestSubstring('abcabcbb')) //console.log("map", map)`