Flatten Array

flatten.js

/** 
 * Takes a nested array and flattens it. Default use is for a nested array of integers
 * But can be used for nested arrays with any type of value by passing in custom mapping and/or filtering functions
 * @param  {Array} arr          The nested array to be flattened
 * @param  {Function} mapIt     Function to map each value
 * @param  {Function} filterIt  Function to filter out unwanted values left over from the mapping
 * @return {Array}              Returns empty array if `arr` is invalid
 */
export default function flatten(arr, mapIt=Number, filterIt=Number.isInteger) {
    let result = []
    if (Array.isArray(arr) && arr.length) {
        result = arr.join().split(',')
        if (typeof mapIt === 'function') result = result.map(mapIt)
        if (typeof filterIt === 'function') result = result.filter(filterIt)
    }
    return result 
}