Non-trivial documentation on a skip list helper method.

gistfile1.coffee

# Private: searches the skiplist in a stairstep descent, following the highest
# path that doesn't overshoot the index.
#
# * next
#   An array that will be populated with the last node visited at every level
#
# Returns the leftmost node whose index is >= the given index
findClosestNode: (index, next) ->
  currentNode = @head
  for i in [@currentLevel..0]
    # move forward as far as possible while keeping the currentNode's index less
    # than the index being inserted
    while currentNode.next[i].index < index
      currentNode = currentNode.next[i]
    # when the next node's index would be bigger than the index being inserted,
    # record the last node visited at the current level and drop to the next level
    next[i] = currentNode
  currentNode.next[0]

In general I like it. A few things points I'd change:

I think the first letter should be capitalized, so searches becomes Searches.

I think next should have a colon trailing it, as the markdown gets folded onto one line which makes it a bit harder to read without it.