wrapSubstring.js

function subMatches(string, subs) {
    return []
        .concat.apply([], subs.map(sub => matchIndexes(string, sub)))
        .reduce((acc, idx) => {
            acc[idx] = true;
            return acc;
        }, Array.from(string).fill(false));
}

function matchIndexes(string, sub, startIndex=0, matches=[]) {
    if((startIndex + sub.length) > string.length) {
        return matches; 
    }

    if(sub === string.substr(startIndex, sub.length)) {
        return matchIndexes(string, sub, startIndex + 1, matches.concat(range(startIndex, sub.length)));
    } else {
        return matchIndexes(string, sub, startIndex + 1, matches);
    }
}

function range(start, count) {
    return Array.from(new Array(count), (n, i) => i + start);
}

function wrapSubs(string, subs) {
    const matches = subMatches(string, subs);
    let open = false;

    return string.split('').map((c, idx) => {
        let ret = '';

        if(matches[idx] && !open) {
            open = !open;
            ret += '<b>';
        } else if(!matches[idx] && open) {
            open = !open;
            ret += '</b>';
        }

        ret += c;

        if(idx === string.length - 1 && open) {
            ret += '</b>';
        }

        return ret;
                
    }).join('');
}

console.log(wrapSubs('abcdxyzzzxzz', ['ab', 'bcd', 'z']));
console.log(wrapSubs('abcdefcd', ['cd']));