SOURCE

function findLongestUnrepeatSubString(S) {
    let start = 0, maxLength = 0;
    const map = new Map();
    for (let i = 0;i < S.length;i++) {
        const letter = S.charAt(i);
        if (map.has(letter)) {
            if (map.size > maxLength) {
                maxLength = map.size;
                start = i - map.size;
            }
            const prev = map.get(letter);
            i = prev;
            map.clear();
            continue;
        }
        map.set(letter, i);
    }
    return S.slice(start, start + maxLength);
}
console.log(findLongestUnrepeatSubString("abackwppa"))