Problem with For Loop

function afficher(tab, n = 10) {
    let output = "";
    for (let i = 0; i < tab.length; i++) {
        if (tab[i] % n == 0) {
            output += "\n";
        }
        output += tab[i] + " ";
    }
    return output;
}

function createArray(nbr, max) {
    let tab = [];
    for(let i = 0; i < nbr; i++) {
        let x = Math.floor(Math.random() * max);
        tab.push(x);
    }
    return tab;
}

let tableau = createArray(6, 10);
console.log(afficher(tableau, 4));

function afficher(tab, n = 10) {
    let output = "";
    for (index in tab) {
        if (index % n == 0) {
            output += "\n";
        }
        output += tab[index] + " ";
    }
    return output;
}

function createArray(nbr, max) {
    let tab = [];
    for(let i = 0; i < nbr; i++) {
        let x = Math.floor(Math.random() * max);
        tab.push(x);
    }
    return tab;
}

let tableau = createArray(6, 10);
console.log(afficher(tableau, 4));