flattenArray.js

(function flatten(arr, currentIndex = 0, flattened = []) {
    let currItem;
    if (currentIndex >= arr.length) {
        return flattened;
    }

    currItem = arr[currentIndex++];

    if (!Array.isArray(currItem)) {
        flattened.push(currItem);
    } else {
        flattened = flattened.concat(flatten(currItem));
    }

    return flatten(arr, currentIndex, flattened);

})([[43,[5,6],7], 10]);