Can't get access to items in array

Tell us what’s happening:
I did only part of task and resursively filled one array with many one-item arrays (it is mergeSort function)
In merge function I’m going to compare and concat pairs of array but before it i don’t get why in resulted array of mergeSort function i have arr[0][0] as 1 - first item from array but arr[1][0], arr[2][0],arr[3][0] etc as undefined

Your code so far

let arr = [];

function mergeSort(a) {
// change code below this line

let hl = parseInt(a.length / 2);
if(a.length > 1){
  let onehalf = a.slice(0,hl);
  let twohalf = a.slice(hl);

// change code above this line
return merge(arr); 

function merge(arr){
if(arr[0][0] != undefined){

  return "Yes";
  return "No";

console.log(mergeSort([1, 4, 423, 21, 10, 2, 8]));

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.130 Safari/537.36.

Challenge: Implement Merge Sort

Link to the challenge:

Please, can someone explain why despite that i have array of one item arrays I can get access only to first number through arr[0][0] - “1” but another arrays return undefined. I don’t know how to find such problem on stackoverflow