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The Official Web Site of Nicholas C. Zakas
Updated: 29 min 33 sec ago

Outputting Markdown from Jekyll using hooks

Mon, 04/15/2019 - 20:00

One of the things I most enjoy about Jekyll1 is writing my blog posts in Markdown. I love not worrying about HTML and just letting Jekyll generate it for me when a post is published. Using Liquid tags directly in Markdown is also helpful, as I can define sitewide or page-specific variables and then replace them during site generation. This is a really useful capability that I wanted to take advantage of to output Markdown for use in other sites. Places like Medium and allow you to post Markdown articles, so I thought repurposing the Markdown I used in Jekyll would make crossposting to those sites easier.

I assumed that there would be a property on the page variable that would give me access to the rendered Markdown, but I was wrong. This began a long journey through relatively undocumented corners of the Jekyll plugin ecosystem. I’m sharing that journey here in the hopes that others won’t have to go through the same frustrating experience.

An introduction to Jekyll plugins

I was relatively unfamiliar with the Jekyll plugin system before trying to figure out how to get the rendered Markdown for a post. Jekyll supports a number of different plugin types2. These plugin types affect Jekyll directly:

  • Generators - plugins that create files. While it’s not required that generators create files, this is the most frequent use case. Plugins like jekyll-archives use generators to create files that wouldn’t otherwise exist.
  • Converters - plugins that convert between text formats. Jekyll’s default of converting Markdown files into HTML is implemented using a converter. You can add support for other formats to be converted into HTML (or any other format) by creating your own converter.
  • Hooks - plugins that listen for specific events in Jekyll and then perform some action in relation to the events.

There are also two plugin types that are used primarily with Liquid:

  • Tags - create a new tag in the format {% tagname %} for use in your templates.
  • Filters - create a new filter that you can use to transform input, such as {{ data | filter }}

There is also a command plugin type that allows you to create new commands to use with the jekyll command line tool. The jekyll build command is implemented using this plugin type.

Designing the solution

My goal was to get the Liquid-rendered Markdown content (so all data processing was complete) for each post into a page property so that I could output that content into a JSON file on my server. That JSON file would then be fetched by an AWS Lambda function that crossposted that content to other locations. I didn’t need to generate any extra files or convert from one format to another, so it seemed clear that using a hook would be the best approach.

Hooks are basically event handlers in which you specify a container and the event to listen to. Jekyll passes you the relevant information for that event and you can then perform any action you’d like. There are four containers you can use with hooks:

  • :site - the site object
  • :page - the page object for each non-collection page
  • :post - the post object for each blog post
  • :document - the document object for each document in each collection (including blog posts and custom collections)

Because I wanted this solution to work for all collections in my site, I chose to use :document as an easy way to make the same change for all collection types.

There were two events for :document that immediately seemed relevant to my goal:

  • :pre_render - fires before the document content is rendered
  • :post_render - fires after the document content is rendered but before the content is written to disk

It seemed clear that getting the Markdown content would require using the :pre_render event, so I started by using this setup:

Jekyll::Hooks.register :documents, :pre_render do |doc, payload| # code goes here end

Each hook is passed its target container object, in this case doc is a document, and a payload object containing all of the relevant variables for working with the document (these are the variables available inside of a template when the document is rendered).

The :document, :prerender hook is called just before each document is rendered, meaning you don’t need to worry about looping over collections manually.

The catch: Rendering doesn’t mean what you think it means

I figured that the content property inside of a :document, :pre_render hook would contain the Liquid-rendered Markdown instead of the final HTML, but I was only half correct. The content property actually contains the unprocessed Markdown that still contains all of the Liquid variables. It turns out that “prerender” means something different than I thought.

The lifecycle of the content property for a given document in Jekyll looks like this3:

  1. The content property contains the file content with the front matter removed (typically Markdown)
  2. :pre_render hook fires
  3. The content property is rewritten with Liquid tags rendered (this is what Jekyll internally refers to as rendering)
  4. The content property is rewritten after being passed through a converter (this is what Jekyll internally refers to as converting)
  5. The :post_render hook fires

While Jekyll internally separates rendering (which is apply Liquid) and converting (the Markdown to HTML conversion, for example), the exposed hooks don’t make this distinction. That means if I want Markdown content with Liquid variables replaced then I’ll need to get the prerendered Markdown content and render it myself.

The solution

At this point, my plan was to create a pre_render hook that did the following:

  1. Retrieved the raw content for each document (contained in doc.content)
  2. Render that content using Liquid
  3. Store the result in a new property called unconverted_content that would be accessible inside my templates

I started out with this basic idea of how things should look:

Jekyll::Hooks.register :documents, :pre_render do |doc, payload| # get the raw content raw_content = doc.content # do something to that raw content rendered_content = doSomethingTo(raw_content) # store it back on the document doc.rendered_content = rendered_content end

Of course, I’m not very familiar with Ruby, so it turned out this wouldn’t work quite the way I thought.

First, doc is an instance of a class, and you cannot arbitrarily add new properties to objects in Ruby. Jekyll provides a data hash on the document object, however, that can be used to add new properties that are available in templates. So the last line needs to be rewritten:

Jekyll::Hooks.register :documents, :pre_render do |doc, payload| # get the raw content raw_content = doc.content # do something to that raw content rendered_content = doSomethingTo(raw_content) # store it back on the document['rendered_content'] = rendered_content end

The last line ensures that page.rendered_content will be available inside of templates later on (and remember, this is happening during pre_render, so the templates haven’t yet been used).

The next step was to use Liquid to render the raw content. To figure out how to do this, I had to dig around in the Jekyll source4 as there wasn’t any documentation. Rendering Liquid the exact same way that Jekyll does by default requires a bit of setup and pulling in pieces of data from a couple different places. Here is the final code:

Jekyll::Hooks.register :documents, :pre_render do |doc, payload| # make some local variables for convenience site = liquid_options = site.config["liquid"] # create a template object template = site.liquid_renderer.file(doc.path).parse(doc.content) # the render method expects this information info = { :registers => { :site => site, :page => payload['page'] }, :strict_filters => liquid_options["strict_filters"], :strict_variables => liquid_options["strict_variables"], } # render the content into a new property['rendered_content'] = template.render!(payload, info) end

The first step in this hook is to create a Liquid template object. While you can do this directly using Liquid::Template, Jekyll caches Liquid templates internally when using site.liquid_renderer.file(doc.path), so it makes sense to use that keep the Jekyll build as fast as possible. The content is then parsed into a template object.

The template.render() method needs not only the payload object but also some additional information. The info hash passes in registers, which are local variables accessible inside of the template, and some options for how liquid should behave. With all of that data ready, the content is rendered into the new property.

This file then needs to be placed in the _plugins directory of a Jekyll site to run each time the site is built.

Accessing rendered_content

With this plugin installed, the Markdown content is available through the rendered_content property, like this:

{{ page.rendered_content }}

The only problem is that outputting page.rendered_content into a Markdown page will cause all of that Markdown to be converted into HTML. (Remember, Jekyll internally renders Liquid first and then the result is converted into HTML.) So in order to output the raw Markdown, you’ll need to either apply a filter that prevents the Markdown-to-HTML conversion from happening, or use a file type that doesn’t convert automatically.

In my case, I’m storing the Markdown content in a JSON structure, so I’m using the jsonify filter, like this:

--- layout: null --- { {% assign post = site.posts.first %} "id": "{{ post.url | absolute_url | sha1 }}", "title": {{ post.title | jsonify }}, "date_published": "{{ | date_to_xmlschema }}", "date_published_pretty": "{{ | date: "%B %-d, %Y" }}", "summary": {{ post.excerpt | strip_html | strip_newlines | jsonify }}, "content_markdown": {{ post.rendered_content | jsonify }}, "content_html": {{ post.content | jsonify }}, "tags": {{ post.tags | jsonify }}, "url": "{{ post.url | absolute_url }}" }

Another option is to create a rendered_content.txt file in the _includes directory that just contains this:

{{ page.rendered_content }}

Then, you can include that file anywhere you want the unconverted Markdown content, like this:

{% include "rendered_content.txt" %} Conclusion

Jekyll hooks are a useful feature that let you interact with Jekyll while it is generating your site, allowing you to intercept, modify, and add data along the way. While there aren’t a lot of examples in the wild, the concept is straightforward enough that with a few pointers, any programmer should be able to get something working. The biggest stumbling point for me was the lack of documentation on how to use Jekyll hooks, so I’m hoping that this writeup will help others who are trying to accomplish similar tasks in their Jekyll sites.

To date, I’ve found Jekyll to be extremely versatile and customizable. Being able to get the Liquid-rendered Markdown (even though it took a bit of work) has made my publishing workflow much more flexible, as I’m now more easily able to crosspost my writing on various other sites.

  1. Jekyll 

  2. Jekyll Plugins 

  3. Jekyll Order of Interpretation 

  4. Jekyll renderer.rb 

Categories: Tech-n-law-ogy

Computer science in JavaScript: Circular Doubly-linked lists

Mon, 03/04/2019 - 19:00

In my previous post, I discussed what changes are necessary to turn a singly linked list into a doubly linked list. I recommend reading that post before this one (if you haven’t already). This post is about modifying a doubly linked list (also called a linear doubly linked list) in such a way that the last node in the list points to the first node in the list, effectively making the list circular. Circular doubly linked lists are interesting because they allow you to continuously move through list items without needing to check for the end of the list. You may encounter this when creating playlists or round-robin distribution of traffic to servers.

It is possible to create a circular singly linked list, as well. I won’t be covering circular singly linked lists in this blog post series, however, you can find source code for a circular singly linked list in my GitHub repo, Computer Science in JavaScript.

The design of a circular doubly linked list

The nodes in a circular doubly linked list are no different than the nodes for a linear doubly linked list. Each node contains data and pointers to the next and previous items in the list. Here is what that looks like in JavaScript:

class CircularDoublyLinkedListNode { constructor(data) { = data; = null; this.previous = null; } }

You can then create a circular doubly linked list using the CircularDoublyLinkedListNode class like this:

// create the first node const head = new CircularDoublyLinkedListNode(12); // add a second node const secondNode = new CircularDoublyLinkedListNode(99); = secondNode; secondNode.previous = head; // add a third node const thirdNode = new CircularDoublyLinkedListNode(37); = thirdNode; thirdNode.previous = secondNode; // point the last node to the first = head; head.previous = thirdNode;

The head of the list and subsequent nodes in the list are created the same way as in a linear doubly linked list. The only difference is the last step where the last node’s next pointer is set to head and the head node’s previous pointer is set to the last node. The following image shows the resulting data structure.

Traversing a circular doubly linked list is a bit different than a linear doubly linked list because following next pointers alone will result in an infinite loop. For example, this is an infinite loop:

let current = head; // infinite loop: `current` is never `null` while (current !== null) { console.log(; current =; }

In some cases you will want to continue iterating over the loop forever, but that typically does not happen in the context of a loop as in this code. In other cases, you’ll want to iterate over each node in the loop until the last node is found. To do that, you’ll need to check to see when current is head, which means you’re back at the beginning of the loop. However, simply swapping null for head in the previous example results in the loop not executing at all:

let current = head; // loop is skipped: `current` is already `head` while (current !== head) { console.log(; current =; }

The problem here is that current started out equal to head and the loop only proceeds when current is not equal to head. The solution is to use a post-test loop instead of a pre-test loop, and in JavaScript, that means using a do-while loop:

let current = head; if (current !== null) { do { console.log(; current =; } while (current !== head); }

In this code, the check to see if current is equal to head appears at the end of the loop rather than at the start. To ensure that the loop won’t start unless current isn’t null, an if statement typically must preceed the do-while loop (you no longer have the pre-test of a while loop to cover that case for you). The loop will proceed until current is once again head, meaning that the entire list has been traversed.

Also similar to linear doubly linked lists, you can traverse the nodes in reverse order by starting from the last node. Circular doubly linked lists don’t separately track the list tail because you can always access the tail through head.previous, for example:

let current = head.previous; if (current !== null) { do { console.log(; current = current.previous; } while (current !== head.previous); } The CircularDoublyLinkedList class

The CircularDoublyLinkedList class starts out looking a lot like the DoublyLinkedList class from the previous article with the exception that there is no tail property to track the last node in the list:

const head = Symbol("head"); class CircularDoublyLinkedList { constructor() { this[head] = null; } }

The primary differences between a linear and circular doubly linked list have to do with the methods for adding, removing, and traversing the nodes.

Adding new data to the list

The same basic algorithm for adding data is used for both linear and circular doubly linked lists, with the difference being the pointers that must be updated to complete the process. Here is the add() method for the CircularDoublyLinkedList class:

class CircularDoublyLinkedList { constructor() { this[head] = null; } add(data) { const newNode = new CircularDoublyLinkedListNode(data); // special case: no items in the list yet if (this[head] === null) { this[head] = newNode; = newNode; newNode.previous = newNode; } else { const tail = this[head].previous; = newNode; newNode.previous = tail; = this[head]; this[head].previous = newNode; } } }

The add() method for the circular doubly linked list accepts one argument, the data to insert into the list. If the list is empty (this[head] is null) then the new node is assigned to this[head]. The extra step to make the list circular is to ensure that both and newNode.previous point to newNode.

If the list is not empty, then a new node is added after the current tail, which is retrieved using this[head].previous. The new node can then be added to Remember, you are actually inserting a new node between the tail and the head of the list, so this operation looks a lot more like an insert than an append. Once complete, newNode is the list tail and therefore must point to this[head] and this[head].previous must point to newNode.

As with a linear doubly linked list, the complexity of this add() method is O(1) because no traversal is necessary.

Retrieving data from the list

The get() method for a circular doubly linked list follows the basic algorithm from the start of this post. You must traverse the list while keeping track of how deep into the list you have gone and ensuring you don’t loop back around to the front of the list. Here is how the get() method is implemented.

class CircularDoublyLinkedList { // other methods hidden for clarity get(index) { // ensure `index` is a positive value and the list isn't empty if ((index > -1) && (this[head] !== null)) { let current = this[head]; let i = 0; do { if (i === index) { return; } current =; i++; } while ((current !== this[head]) && (i <= index)); } return undefined; } }

The get() method first checks to make sure that index is a positive value and that the list isn’t empty. If either case is true, then the method returns undefined. Remember, you must always use an if statement to check if a circular doubly linked list is empty before starting a traversal due to the use of a post-test instead of a pre-test loop.

Using the same traversal algorithm as discussed earlier, the get() method uses the i variable to track how deep into the list it has traversed. When i is equal to index, the data in that node is returned (existing the loop early). If the loop exits, either because it has reached the head of the list again or index is not found in the list, then undefined is returned.

As with a linear doubly linked list, the get() method’s complexity ranges from O(1) to O(n);

Removing data from the list

Removing data from a circular doubly linked list is basically the same as with a linear doubly linked list. The differences are:

  1. Using a post-test loop instead of a pre-test loop for the traversal (same as get())
  2. Ensuring that the circular links remain on the head and tail nodes when either is removed

Here is what the implementation of a remove() method looks like:

class CircularDoublyLinkedList { // other methods hidden for clarity remove(index) { // special cases: no nodes in the list or `index` is an invalid value if ((this[head] === null) || (index < 0)) { throw new RangeError(`Index ${index} does not exist in the list.`); } // save the current head for easier access let current = this[head]; // special case: removing the first node if (index === 0) { // if there's only one node, null out `this[head]` if ( === this[head]) { this[head] = null; } else { // get the last item in the list const tail = this[head].previous; /* * Set the tail to point to the second item in the list. * Then make sure that item also points back to the tail. */ =; = tail; // now it's safe to update the head this[head] =; } // return the data at the previous head of the list return; } let i = 0; do { // traverse to the next node current =; // increment the count i++; } while ((current !== this[head]) && (i < index)); // the node to remove has been found if (current !== this[head]) { // skip over the node to remove =; = current.previous; // return the value that was just removed from the list return; } // `index` doesn't exist in the list so throw an error throw new RangeError(`Index ${index} does not exist in the list.`); } }

While there are special cases in this remove() method, almost every case requires adjusting pointers on two nodes due to the circular nature of the list. The only case where this isn’t necessary is when you are removing the only node in the list.

Removing the first node in the list (index is 0) is treated as a special case because there is no need for traversal and this[head] must be assigned a new value. The second node in the list becomes the head and it previous pointer must be adjusted accordingly.

The rest of the method follows the same algorithm as for a linear doubly linked list. As we don’t need to worry about the special this[head] pointer, the search for and removal of the node at index can proceed as if the list was linear.

You can further simply removal of nodes if you don’t mind losing track of the original head of the list. The implementation of CircularDoublyLinkedList in this post assumes you want the original head of the list to remain as such unless it is removed. However, because the list is circular, it really doesn’t matter what nodes is considered the head because you can always get to every other node as long as you reference to one node. You can arbitrarily reset this[head] to any node you want an all of the functionality will continue to work.

Creating iterators

There are two distinct use cases for iterators in a circular linked list:

  1. For use with JavaScript’s builtin iteration functionality (like for-of loops)
  2. For moving through the values of the list in a circular fashion for specific applications (like a playlist)

To address the first case, it makes sense to create a values() generator method and a Symbol.iterator method on the class as these are expected on JavaScript collections. These methods are similar to those in a doubly linked list with the usual exceptions that the loop must be flipped and the you need to check to see if you’ve reached the list head to exit the loop. Those two methods look like this:

class CircularLinkedList { // other methods hidden for clarity values() { // special case: list is empty if (this[head] !== null) { // special case: only one node if (this[head].next === this[head]) { yield this[head].data; } else { let current = this[head]; do { yield; current =; } while (current !== this[head]); } } } [Symbol.iterator]() { return this.values(); } }

The values() generator method has two special cases: when the list is empty, in which case it doesn’t yield anything, and when there is only one node, in which case traversal isn’t necessary and the data stored in the head is yielded. Otherwise, the do-while loop is the same as the one you’ve seen through this post.

Creating an iterator that loops around is then just a matter of modifying this algorithm so the loop never exits. Here is what that looks like:

class CircularDoublyLinkedList { // other methods hidden for clarity *circularValues() { // special case: list is empty if (this[head] !== null) { let current = this[head]; // infinite loop do { yield; current =; } while (true); } } }

You wouldn’t want to use the circularValues() generator method in any situation where JavaScript will drain an iterator (as in the for-of loop) because this will cause an infinite loop and crash. Instead, manually call the next() method of the iterator whenever you need another value.

For this method, it really doesn’t matter if you use a do-while loop or a while loop. I used do-while to keep it consistent with the rest of this post, but you can use any flavor of infinite loop that you want.

Using the class

Once complete, you can use the circular doubly linked list implementation like this:

const list = new CircularDoublyLinkedList(); list.add("red"); list.add("orange"); list.add("yellow"); // get the second item in the list console.log(list.get(1)); // "orange" // print out all items for (const color of list.values()) { console.log(color); } // remove the second item in the list console.log(list.remove(1)); // "orange" // get the new first item in the list console.log(list.get(1)); // "yellow" // convert to an array const array1 = [...list.values()]; const array2 = [...list]; // manually cycle through each item in a circular manner const iterator = list.circularValues(); let { value } =; doSomething(value); ({ value } =; doSomething(value);

The full source code is available on GitHub at my Computer Science in JavaScript project.


Circular doubly linked lists are setup in a similar manner as linear doubly linked lists in that each ndoe has a pointer to both the next and previous nodes in the list. The difference is that the list tail always points to the list head so you can follow next pointers and never receive null. This functionality can be used for applications such as playlists or round-robin distribution of data processing.

The implementation of doubly linked list operations differs from linear doubly linked lists in that you must use a post-test loop (do-while) to check if you’re back at the beginning of the list. For most operations, it’s important to stop when the list head has been reached again. The only exception is in creating an iterator to be called manually and which you’d prefer never ran out of items to return.

The complexity of circular doubly linked list operations is the same as with linear doubly linked list operations. Unlike the other data structures discussed in this blog post series, circular doubly linked lists can be helpful in JavaScript applications that require repeating cycling through the same data. That is one use case that isn’t covered well by JavaScript’s builtin collection types.

Categories: Tech-n-law-ogy