function solution(participant, completion) {
    for (leti = 0; i < participant.length; i++) {
        if (completion.includes(participant[i])) {
            let value = participant[i];
            participant.splice(i, 1);
            completion.splice(completion.indexOf(value), 1);
            i--;
        }
    }
    return participant.join('');
}

let participant = ['marina', 'josipa', 'nikola', 'vinko', 'filipa'];
let completion = ['josipa', 'filipa', 'marina', 'nikola'];

console.log(solution(participant, completion))