SOURCE

function flat(arr, depth = 1) {
    let result = [];
    arr.forEach(item => {
        if(Array.isArray(item) && depth >0) {
            result = result.concat(flat(item, depth-1));
        } else {
            result.push(item)
        }
    });
    return result;
}

const arr = [1,[2,[3,[4,5]]],6];
console.log(flat(arr,2))