Longest Substring Without Repeat Characters

var lengthOfLongestSubstring = function(s) {
    if (s === '') {
        return 0;
    }

    var lenMax = 1,
        lenCurr = 1,
        i, repeat;

    for (i = 1; i < s.length; i++) {
        repeat = s.substr(i - lenCurr, lenCurr).indexOf(s.substr(i, 1));

        if (repeat == -1) {
            lenCurr++;
        } else {
            lenCurr -= repeat;
        }

        if (lenCurr > lenMax) {
            lenMax = lenCurr;
        }
    }

    return lenMax;
};