How to find longest substring without repeating characters in JavaScript?

7bbszl.js

const lengthOfLongestSubstring = s => {
  let longest = 0;
  let start = 0;
  const seen = {};

  [...s].forEach((char, i) => {
    if (char in seen && start <= seen[char]) {
      longest = Math.max(i - start, longest);
      start = seen[char] + 1;
    }

    seen[char] = i;
  });

  return Math.max(s.length - start, longest);
};
//https://codereview.stackexchange.com/questions/220228/longest-substring-without-repeating-characters-in-js