var number_list =  [[1, 2, [3]], 4];

function flatten(n_list){
    // empty final list to storage the numbers
    var flattened_list = [];
    // for each list item, if item type if a number, add to the flattened_list, else,
    // iterate over each element of the array doing the same
    var func = (x) => (typeof(x) == "number") ? flattened_list.push(x) : x.map((t) => func(t)) ;

    func(n_list);

    return flattened_list;
}

console.log(flatten(number_list));