Partition On

function partitionOn(pred, items) {
  var falsies = items.filter(function (num) {return !pred(num)}),
      truths = items.filter(pred),
      newArr = falsies.concat(truths);
  for (var i = 0; i < newArr.length; i++) items[i] = newArr[i];
  return falsies.length;
}