Flatten Arbitrarily Nested Arrays Method (Javascript)

flattenArray.js

// Using recursion this method will concatenate the values of nested arrays into a single array
function flatten(arr) {
    var holder = [];
    for(var i = 0; i < arr.length; i++) {
        if(Array.isArray(arr[i])) {
            holder = holder.concat(flatten(arr[i]));
        } else {
            holder.push(arr[i]);
        }
    }
    return holder;
}

flatten([[1,2,[3]],4]) // [1, 2, 3, 4]