Bonfire: Chunky Monkey

function chunk(arr, size) {

  var multi = [];
  i = 0;
  
  while (i < arr.length){
    multi.push(arr.slice(i,i+=size));
  }
  return multi;
}

chunk(["a", "b", "c", "d"], 1);

Here an alternative where I extracted the increment i=i+size, for more readability (maybe?):

function chunk(arr, size) {

  var multi = [];
  i = 0;
  
  while (i < arr.length){
    multi.push(arr.slice(i,i+size));
    i=i+size;
  }
  return multi;
}

chunk(["a", "b", "c", "d"], 1);

Same with a for loop instead of while:

function chunk(arr, size) {

  var multi = [];
  for (i=0;i < arr.length;i+=size){
    multi.push(arr.slice(i,i+size));
  }
  return multi;
}

chunk(["a", "b", "c", "d"], 2);