flatten.js

/**
 * Hight-Order Funcion
 * Returns a single item by iterating through the list, successively calling the iterator function and passing it an accumulator value and 
 * the current value from the array, and then passing the result to the next call.
 * @param  {Array}   list Array of element
 * @param  {Function} fn   Functor. The iterator function. Receives two values, the accumulator and the current element from the array.
 * @param  {*}   acc  The accumulator value.
 * @return {*}        The final, accumulated value.
 */
const reduce = (list, fn, acc) => { 

    for(let i of list) { 
        acc = fn(acc, i) 
    } 

    return acc 
}

/**
 * Returns a new list by pulling every item out of it (and all its sub-arrays) and putting them in a new array, depth-first.
 * Recursion aproach
 * @param  {Array} list The array to consider.
 * @return {Array}       The flattened list.
 */
const flatten = list => reduce(list, (acc, el) => el instanceof Array ? acc.concat( flatten(el) ) : acc.concat([ el ]), [] )



/**
 * Returns a new list by pulling every item out of it (and all its sub-arrays) and putting them in a new array.
 * O(n) fast flatten aproach
 * @param  {Array} list The array to consider.
 * @return {Array}      The flattened list.
 */
const  fast_flatten = list => {
    
    let ret = []

    for(let el; el = list.pop(); ){
        
        if (el instanceof Array) {
            list.push.apply(list, el)
        } else {
            ret.push(el)
        }
    }

    ret.reverse()
    return ret
}

flatten([ [1,2, [3 ]], 4])
//[ 1, 2, 3, 4 ]
fast_flatten([ [1,2, [3 ]], 4])
//[ 1, 2, 3, 4 ]