Flatten array of arrays

flatten.js

// returns a flattened array
function flattenArray(arr = []) {
        return arr.reduce((prev, value) => {
                if (Array.isArray(value)) {
                        // make a recursive call to flatten the inner array
                        return [...prev, ...flattenArray(value)];
                }
                return [...prev, value];
        }, []);
}