Flatten nested arrays

nestedFlatten.js

const n = [1,[2,[3,4,5]],6,[7,[8],[9,[10]]]];
const n2 = [1,[2,[3,[4,[5],6],7],8],9];

/**
 * Flattens arbitrarilily nested arrays. Pulls off the first item (a) from the array
 * and preserves the rest (arr) using the spread operator. 
 * 
 * @param {Array} [] The nested array to flatten
 * @param {Array} flat The nested array to flatten
 * @returns {Array} Recursively returns a flattened array
 */
const flatten = (arr, flat=[]) => { 
  arr.forEach(a => Array.isArray(a) ? flatten(a, flat) : flat.push(a));
  return flat;
}


/**
 * Flattens arbitrarilily nested arrays. Pulls off the first item (a) from the array
 * and preserves the rest (arr) using the spread operator. 
 * 
 * @param {Array} [] The nested array to flatten
 * @returns {Array} Recursively returns nested arrays deepest -> shallow
 */
const flatten2 = ([a, ...arr]) => a ? [...(Array.isArray(a) ? flatten2(a) : [a]), ...flatten2(arr)] : [];

/*
 * The spread operator in flatten2 is basically the same exact idea of arr.forEach(a... in flatten. 
 * It picks off the first item and then returns the rest using the reversed spread operator. Similar in
 * a way to Array.shift().
 */

console.log(flatten(n));
console.log(flatten(n));

// Result should be:
// Array(10) [1,2,3,4,5,6,7,8,9,10]
// Array(10) [1,2,3,4,5,6,7,8,9,10]

console.log(flatten(n2));
console.log(flatten(n2));

// Result should be:
// Array(9) [1,2,3,4,5,6,7,8,9]
// Array(9) [1,2,3,4,5,6,7,8,9]