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var old_array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]

function arrayDividedBy(n, old_array) {

  var new_array = [];

  while (old_array.length) {
    if (old_array.length >= n) {
      new_array.push(old_array.splice(0, n));
    } else {
      new_array.push(old_array.splice(0, old_array.length));
    }
  }

  return new_array;
}

console.log(arrayDividedBy(4, old_array))