split-on-change.js

var input = [1, 1, 1, 2, 2, 2, 3, 3];
var outputExpected = [[1, 1, 1], [2, 2, 2], [3, 3]];

var outputActual = input.reduce(function(acc, currentItem) {
  var accLength = acc.length;
  var currentGroup = acc[accLength - 1];
  var currentGroupLength = currentGroup.length;
  if (currentGroupLength > 0) {
    var previousItem = currentGroup[currentGroupLength - 1];
    if (previousItem === currentItem) {
      currentGroup.push(currentItem);
    } else {
      acc.push([currentItem]);
    }
  } else {
    currentGroup.push(currentItem);
  }
  return acc;
}, [[]]);