First Unique Character in a String - LeetCode javascript solutions

Problem

Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.

Example 1:

Input: s = "leetcode"
Output: 0

Example 2:

Input: s = "loveleetcode"
Output: 2

Example 3:

Input: s = "aabb"
Output: -1

Constraints:

1 <= s.length <= 105
s consists of only lowercase English letters.

Solution

/**
 * @param {string} s
 * @return {number}
 */
var firstUniqChar = function(s) {
    var map = {};
    for (var i = 0; i < s.length; i++) {
        map[s[i]] = (map[s[i]] || 0) + 1;
    }
    for (var i = 0; i < s.length; i++) {
        if (map[s[i]] === 1) return i;
    }
    return -1;
};

Explain:

nope.

Complexity:

Time complexity : O(n).
Space complexity : O(1).