flattenArray.js

// Flattens an array, e.g. [[1,2,[[[3]]]],4] => [1, 2, 3, 4]
var flattenArray = (array) => {
    let initialValue = [];
    return array.reduce((result, ele) => {
        if (Array.isArray(ele)) {
           let flat = flattenArray(ele);
           return result.concat(flat);
        } else {
           result.push(ele);
           return result;
        }
    }, initialValue);
};