Array chunking algorithm - Javascript

Working with arrays in Javascript.

[МихаилЛобанов-бз]

function chunk(arr, num){
const chunked = [];
for(let i = 0; i<arr.length; i+=num){
chuncked.push(arr.slice(i, i+num));
}
return chunked;
}

[youngbinpaek]

Having some trouble wrapping my head around line 15, for some reason I really want to chunk.push(elem) and not last.push(elem). Could you explain a little why it's last.push(elem)? I think I'm getting confused because I look at line 11 where last is declared. Thanks!

[Hasansaid]

Would this be O(n)? Since you have to go thru each element in the array right?