Function that will flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].

flattenArray.js

/**
 * Flattens passed in array.
 *
 * @param {Array} input is the array to flatten.
 * @returns {out} returns flattened array.
 */

function flatten(input) {
    let i, 
        placeHolder = [input], 
        lastIndexSaved = [-1], 
        toBeout = [];
    while (placeHolder.length) {
        input = placeHolder.pop();
        i = lastIndexSaved.pop() + 1;
        for (; i < input.length; ++i) {
            if (Array.isArray(input[i])) {
                placeHolder.push(input);
                lastIndexSaved.push(i);
                input = input[i];
                i = -1;
            } else toBeout.push(input[i]);
        }
    }
    return out;
}