flatter for array

flatter.js

function flatter(arr) {
    return arr.flat(Infinity);
}

flatter(
    [1, 2, [3, 4, [5]], 6, [[7, [8]], 9]],
);

function flatter2(arr) {
    const arrCopy = arr.slice();
    const result = [];
    while (arrCopy.length) {
        const next = arrCopy.pop();
        if (Array.isArray(next)) {
            arrCopy.push(...next);
        } else {
            result.push(next);
        }
    }
    return result.reverse();
}

flatter2([1, 2, [3, 4, [5]], 6, [[7, [8]], 9]]);