AI solution for JavaScript Coding Question on BFE.dev
105. find the first duplicate character in a string

One possible solution in JavaScript:

function firstDuplicate(str) {  let charSet = new Set();  for(let i = 0; i < str.length; i++) {    let char = str[i];    if (charSet.has(char)) {      return char;    } else {      charSet.add(char);    }  }  return null;}

This solution uses a set to keep track of unique characters seen so far. As soon as a duplicate character is found, it is returned. If no duplicates are found, the function returns null. The time complexity of this solution is O(n) and the space complexity is O(n), since we need to store all unique characters in the set.

A more memory-efficient solution would be to use an object to keep track of character frequencies instead of storing all unique characters in a set. This would reduce space complexity to O(1), but increase time complexity to O(n^2). Here's the implementation:

function firstDuplicate(str) {  let charFreq = {};  for(let i = 0; i < str.length; i++) {    let char = str[i];    if (charFreq[char]) {      return char;    } else {      charFreq[char] = true;    }  }  return null;}

Overall, the choice of solution depends on the trade-off between time and space complexity that is acceptable for the given use case.